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XVIII; The Evolution of Species

The Evolution of Species

The general notion of evolution that in some sense the present state of things has arisen from earlier states by some kind of development is as old as recorded thought; and traces of the application of the idea to the case of the evolution of living organisms by a process of growth and modification of primordial germs are to be found in the utterances of the earliest Greek thinkers. Even the idea of the survival of the fittest was indicated in a crude form by Empedocles (495–435 B.C.) who depicted the gradual origin in accordance with abiogenesis first of plants and then of animals through the chance play of combining and separating forces acting on the elements fire water earth and air. After elimination of the earliest forms fitter ones were he thought produced but still fortuitously. Both the atomists typified by Democritus and their rivals such as Anaxagoras who entertained teleological conceptions had some general ideas as to the occurrence of fitness or adaptation. With Aristotle the notion of evolution attained to greater precision than with his predecessors. He followed Plato in regarding the creation of the cosmos as a process of descent from the more to the less perfect but his dualistic interpretation of this process permitted him to hold a teleological theory of organic evolution. Although he regarded types as the realization of an original formative principle and in this sense fixed he appears to have entertained the possibility of the spontaneous generation of the lowest organisms. He regarded organs as fashioned by nature in the order of their necessity those essential to life coming first. Aristotle clearly stated the conception of the survival of the fittest but only to reject it. He definitely refused to admit that adaptation is due to the elimination of the unfit but regarded the process of successive adaptation as due to an immanent principle striving to attain a certain end—and that end he believed to be the production of man!

During the centuries between the close of the Greek period and the Renaissance under the influence of the Church the theory of special creation of species resting upon a literal interpretation of the Mosaic cosmogony became dominant. But there were not wanting among the Fathers of the Church and later among the Scholastics those who like St Augustine by accepting a less literal interpretation combined it with the general conception of evolution. The following utterance1 of St Augustine in favour of freedom of scientific thought is of much interest as showing how far he was in advance of the later attitude of the Church for many centuries in this matter.
It very often happens (he writes) that there is some question as to the earth or the sky or the other elements of this world... respecting which one who is not a Christian has knowledge derived from most certain reasoning or observation and it is very disgraceful and mischievous and of all things to be carefully avoided that a Christian speaking of such matters as being according to the Christian Scriptures should be heard by an unbeliever talking such nonsense that the unbeliever perceiving him to be as wide from the mark as east from west can hardly restrain himself from laughing.
Unfortunately this more liberal interpretation of the Mosaic cosmogony died out amongst theologians and the doctrine of special creation became completely dominant until the nineteenth century. When it was discovered that many species had become extinct and been replaced by new ones the doctrine was extended to embrace a whole series of special creations of species.
In the period immediately after the Renaissance the attention of scientific workers was devoted rather to detailed work in Anatomy and Physiology than to the working out of so general a conception as that of organic evolution. It was not until the seventeenth century that largely in the hands of Philosophers the notion of organic evolution as a speculative idea again came into some prominence in the modern world. One of the first to suggest the transmutation of species by accumulated variations and to advocate the experimental investigation of the subject was Francis Bacon (1561–1626). Descartes (1596–1650) although he was much hampered in the expression of his opinions by what he regarded as the necessity of paying lip-service to the orthodox doctrine of special creation clearly showed in his Traité de l'homme and in his essay Sur les passions that he believed himself to have found an explanation of the universe and in particular of the phenomena of life on purely physical principles and also of the mode in which the universe had been evolved. Leibniz in his doctrines of monadism and of continuity developed the view that each monad is the focus of an endless process of evolution and involution; and that all natural orders of beings form a single chain along which progress is made continuously. Spinoza also expressed a belief in evolution in accordance with invariable laws. Immanuel Kant should also be mentioned as having at least in his earlier period advanced the conceptions of Selection Adaptation Environment and Inheritance; but he appears later to have abandoned his evolutionary views as unconfirmed by observation.
The first Naturalist who clearly expressed the idea that the unity of plan in the structure of animals may be due to community of origin was Buffon (1707–1788). His statements on the subject are vacillating perhaps owing to difficulties which he felt in breaking with the orthodox conceptions and he had no complete or consistent theory of evolution; but some of his utterances are quite explicit as to the possibility of the descent of all the species of animals from one stock. For example in his account of the Ass he argues that: “Once admit that within the bounds of a single family one species may originate from the type species by degeneration then one might reasonably suppose that from a single being Nature could in time produce all the other organized beings.” Elsewhere he speaks of the reduction of the two hundred species of quadrupeds which he described to a small number of families “from which it is not impossible that all the rest are derived.” Within each of the families the species branches off from a parent species. Buffon anticipated various ideas such as pangenesis the struggle for existence artificial and natural selection and geographical isolation; all of which became later of fundamental importance in the theory of evolution. Being originally in accord with Linnaeus his views gradually changed but there were in his statement of them continual indications of indecision as regards the opposed conceptions of special creation and evolution.
The great systematist Linnaeus (1707–1778) reckoned at first “as many species as issued in pairs from the hands of the Creator” but later he modified his views so far as to admit that although “all the species of one genus constituted at first one species they were subsequently multiplied by hybrid generation that is by intercrossing with other species.”
It is remarkable that Cuvier (1769–1832) although he recognized very clearly a succession of forms in time was a most resolute opponent of the evolutionary view of descent mainly on account of lack of sufficient evidence for it. The course of the changes in his opinions was diametrically opposite to that of Buffon and Linnaeus. Starting with the most advanced views of Buffon as to mutability of species he gradually arrived at a point such as that from which Linnaeus started insisting on the fixity not only of species but also of varieties. Although evolutionary ideas had some influence over the minds of such men as Charles Bonnet (1720–1793) Robinet (1735–1820) Oken (1779–1851) and Geoffroy Saint-Hilaire none of them developed a definite and comprehensive theory of the matter. Some of them such as Bonnet and Oken appear to have regarded evolution as a purely ideal conception of the development of types of structure and not to be regarded as an historical account of the genesis of species. Others such as Robinet Treviranus Tiedemann and Meckel held the view that it described an actual historical process. Under the influence of such ideas as that of the scale of beings evolution was most frequently regarded as taking place through a linear series as it were along a straight ladder and not as figured by a genealogical tree as it came to be by Lamarck and Darwin and by all later evolutionists. The necessary preliminaries of a theory of evolution based upon scientific induction and not merely upon philosophical speculation consisted in the development of the studies of Embryology Palaeontology Comparative Anatomy and Distribution. This preliminary work was carried out with much vigour in the eighteenth century but not until the nineteenth was it sufficiently completed to provide a sufficiently solid basis of facts and ideas.
Jean Baptiste Pierre Antoine de Monet known as the Chevalier de Lamarck (1744–1829) first a soldier then a medical man commenced his investigations as a Botanist and afterwards passed on to Zoology. He did important work in systematic Botany and Zoology; and these studies led him to the conception of the mutability of species and to the theory of the origin of species by descent. In this domain he may be regarded as the most important figure previous to Darwin; and certain of his opinions which differ from those of Darwin have not lost their importance in relation to the more recent discussions of the problems of evolution. In one of his earliest works written in 1766 presented to the Paris Academy of Sciences in 1780 and published in 1704 he affirmed his belief in the immutability of species and his disbelief in abiogenesis; he asserted that all organic individuals descend from other individuals entirely similar which taken together constitute the whole species. His change of opinion did not become manifest until 1802 when he sketched out his evolution theory in his Recherches sur l'organisation des corps vivants. About the same time he proposed the term “Biology” for the sciences of life; it is remarkable that in the same year an identical proposal was made by Treviranus also one of the pioneers of the theory of evolution largely from the point of view of the Philosophy of Nature. The highest point of development of Lamarck's views was exhibited in his Philosophie Zoologique published in 1809; in which his whole scheme of Evolution is gradually built up in logical form. His scheme includes the main ideas that species vary under changing external influences; that there is a fundamental unity in the animal kingdom; and that there exists a progressive and perfecting development. Lamarck's theory of evolution is closely allied in some points with the theory given somewhat earlier by Erasmus Darwin the grandfather of Charles Darwin. The fundamental assumption made by Lamarck is that changes acquired by means of the functional reactions of an animal with its environment and changes produced in a plant by means of changes in its environment are transmissible by heredity to the offspring. He believed that changes in the environment produce no direct changes in the animal organism but that the indirect changes are due to induced changes of habit or of functional reaction of the animal. The changed habits involving the new or changed use of parts result in structural changes which are transmissible to offspring and thus form the main factor in evolution. In the case of plants changes of soil and climate have a direct influence upon the structure; and the changes so produced are transmissible.
The essence of Lamarck's theory is contained in four laws; the law of growth; the law of functional reaction; the law of use and disuse; and the law of use-inheritance. In accordance with the law of growth the size of any living body and the dimensions of its parts are increased by the body's own activities up to some limit imposed by the nature of the living body itself. The law of functional reaction asserts that the production of a new organ results from a new need which continues to be felt and from the new movements originated and sustained by this need. The law of use and disuse asserts that the development and power of action of organs are in proportion to the use made of those organs. The law of use-inheritance asserts that all that has been acquired imprinted or changed in the organization of an individual during the course of its life is transmitted to the new individuals that descend from the individual so modified. The difficulty of understanding the exact nature of Lamarck's conceptions is much increased by the fact that although he propounded the most thorough-going materialistic ideas his view of life and evolution is expressed in psychological terms; and thus contradictions arise within his theory owing to the irreconcilability of his materialism with his vitalism. His apparently materialistic conceptions are expressed in the statements:
No kind or particle of matter can have in itself the power of moving living feeling thinking nor of having ideas; and if outside of man we observe bodies endowed with all or one of these faculties we ought to consider these faculties as physical phenomena which Nature has been able to produce not by employing some particular kind of matter which itself possesses one or other of these faculties but by the order and state of things which she has constituted in each organization and in each particular system of organs... Every animal faculty of whatever nature it may be is an organic phenomenon and results from a system of organs or an organ-apparatus which gives rise to it and upon which it is necessarily dependent... The more highly a faculty is developed the more complex is the system of organs which produces it and the higher the general organization; the more difficult also does it become to grasp its mechanism. But the faculty is none the less a phenomenon of organization and for that reason purely physical.
Lamarck regarded evolution as a process of gradual complication of organisms by which new organs and therefore new faculties arise. On the other hand he divided animals into three groups; insensitive animals; sensitive animals; and intelligent animals. The first group have no principle of reaction to external excitations but passively prolong them into actions in accordance with purely mechanical principles; they possess merely irritability. The second group have in addition to irritability a power which Lamarck calls the “sentiment intérieur.” This is a power of reaction to external stimuli which involves the feeling of a need and results in instant action; it is usually called instinct in animals and does not imply either consciousness or will but acts by transformation of external into internal excitations. The third group consisting of vertebrates have the faculties of intelligence and will in addition to the irritability of the other two groups and the “sentiment intérieur” possessed by the second group. In Lamarck's opinion intelligence and will have little or nothing to do with evolution. The “sentiment intérieur” produces not only instinctive actions but also the formation of new organs as due to needs experienced by this inner feeling. It is not easy to apprehend the precise meaning which Lamarck attached to his first law that of growth. The extension is not produced simply by nutrition an immanent power is required to produce it; and this power he regards as dependent on a subtle fluid somewhat in accordance with the ancient conception of a soul consisting of a very subtle form of matter. In his second law that of functional reaction the psychological conception of a felt need is the essential factor; and this it would appear cannot be reconciled with his materialistic conceptions unless indeed it is capable of some other interpretation; for although the need is not a conscious one it is felt by the “sentiment intérieur.” Moreover at this point a definite hiatus appears between the case of animals of the lowest group and those of the two higher groups. For animals of the insensitive kind possess no “sentiment intérieur” which can experience needs; in their case their behaviour and consequently their evolution depends upon the purely mechanical action of fluids set in motion by the direct physical action of the environment; whereas as Lamarck states “this is not the case with the more highly organized animals which possess feeling.” As soon as a need is felt the inner feeling directs the fluids and forces to the part of the body which can by its action satisfy the need. If the requisite organ exists it is stimulated to action; if no such organ exists and the need is of a sustained character the required organ is gradually produced and developed in accordance with the third law that of use and disuse.
It is in the third and fourth laws which admit of verification or refutation by observation that the main present interest of Lamarck's theory lies. The third law asserts the priority of function to form and the fourth law asserts the heritability of acquired characters a proposition which has become in our day one of the most keenly debated questions. Great as was the work of Lamarck as a Naturalist his reputation as a man of Science has suffered owing to his unrestrained tendency towards speculation of a philosophical kind.
In the decades immediately preceding the new epoch which commenced with the publication of Darwin's Origin of Species the idea of the derivation of species from one another or from primordial forms was familiar to Biologists but no general theory of the evolution of species was developed. By such men as Cuvier and von Baer the evidence in favour of these current notions of evolution was regarded as insufficient; although in 1834 von Baer expressed his belief in a limited amount of evolution. The facts of variability and those of palaeontology led him to believe that many species have been evolved from parent stocks; but the absence of sufficient evidence led him to reject any comprehensive doctrine of descent from a primordial stock. Both in England and in France the fixity of species was the creed of the great majority of Biologists including Richard Owen who never accepted Darwinian views; they spent their energies in the detailed study of special departments of Biological Science without taking much interest in any wide generalizations. Those of them who had read the works of Lamarck were for the most part unconvinced. The French Naturalist Geoffroy Saint-Hilaire a colleague of Lamarck who was by nature more a Philosopher than a Naturalist belonged more to the Buffon-Lamarck school of thought than to that of Cuvier the great opponent of the evolutionary idea although Saint-Hilaire held views very different from those of Lamarck as to the principal factors in evolution and was less radical in his views. Saint-Hilaire denied the inherited influences of habit which Lamarck regarded as all-important and considered that the transformations of organisms were due to the direct influence of the environment the role of the organisms being comparatively passive. In the celebrated debate between him and Cuvier which was held in the Academy of Sciences in 1830 the superior knowledge of Cuvier gave the victory to the anti-evolutionist side; the basis of fact necessary to build up a solid theory of evolution by induction being at that time insufficient. It is interesting to remark that Goethe who was himself a philosophical evolutionist expressed much greater interest when he received the news of this discussion than he did as regards the news of the Paris revolution which arrived at the same time.
Robert Chambers published his well known work Vestiges of the Natural History of Creation in 1844. The evolutionary views which it contained led to its being badly received by reviewers who charged the author with irreligious tendencies. Like Saint-Hilaire and Buffon Chambers regarded modifications of animal structure as due to the direct action of the environment in opposition to the opinion of Lamarck; but he combined with this the Aristotelian conception of a perfecting principle.
The state of opinion in the period just before the Darwinian has been described by Weismann in his Descendenz-Theorie; he writes1:
It is impossible to estimate the effect of Darwin's book on the Origin of Species unless we fully realize how completely the biologists of that time had turned away from general problems. I can only say that we who were then the younger men studying in the fifties had no idea that a theory of evolution had ever been put forward for no one spoke of it to us and it was never mentioned in a lecture. It seemed as if all the teachers in our Universities had drunk of the waters of Lethe and had utterly forgotten that such a theory had ever been discussed or as if they were ashamed of these philosophical flights on the part of Natural Science and wished to guard their students from similar deviations.
It should however be observed that there were some exceptions to this attitude of indifference to general conceptions. In 1830 there appeared Lyell's Principles of Geology in which the author rejected the notion of Cuvier that sudden catastrophes have been an important element in producing terrestrial changes. Various palaeontological discoveries from the time of Cuvier paved the way for the conception of evolution. From a philosophical point of view Herbert Spencer in 1852 advocated the transmutation theory. But neither the cell-theory nor the great advance of knowledge of Embryology succeeded in overcoming the hostility of the Naturalists of the period to such ideas and indeed to the discussion of any general conceptions.
The inception of the idea of theory of Descent in the mind of Darwin was due to the varied observations he made during the five years of the voyage of the Beagle to South America commenced in 1831. He took with him Lyell's Principles of Geology in which Lamarck's doctrines are fully discussed. He himself stated in later years that three classes of facts had brought the matter strongly before his mind; the manner in which closely allied species replace species in going southward; the close affinity of the species inhabiting the islands near South America to those proper to the continent; and the relation of certain living species to the extinct species. From 1835 onwards Darwin devoted his time to meditation on the theory and mode of transmutation and to the collection and sifting of an enormous mass of facts especially relating to domestic animals and cultivated plants until then mostly ignored by scientific men. The idea of the survival of the fittest was first suggested to him by a study of Malthus' Principles of Population. Malthus had pointed out that a competition amongst individuals arises in relation to the food supply from the fact that man tends to increase in geometrical ratio whilst the increase of the food supply is only in arithmetical ratio; thus bringing about the disappearance of individuals less suited than others to sustain the contest. Darwin saw that an extension of this idea might explain the adaptation of all living organisms to the environment. Until 1858 Darwin published no account of his new theory; he was determined to hold it back until the verification from fact should afford evidence of irresistible weight. In that year Alfred Russel Wallace who afterwards became his friend had independently reached a similar theory and the result of a communication by Wallace of his own manuscript embodying his views to Darwin was that two short papers were published in the Journal of the Linnean Society on June 30 1858; the first by Darwin consisted of an abstract of manuscripts written in 1839 and 1844: “On the variation of Organic Beings in a state of Nature; on the Natural Means of Selection; on the Comparison of Domestic Races and True Species”; together with a letter written in 1857 to as a Gray. The second paper by Wallace consisted of an essay written in February 1858 “On the Tendency of Varieties to depart independently from the Original Type.” There is much similarity together with striking differences in the views therein expressed by the two investigators.
The publication of the Origin of Species in 1859 followed in 1871 by the Descent of Man had an effect upon Biological Science and ultimately upon other departments of Science such as Sociology and Anthropology which can only be described as revolutionary. This was not due mainly to the novelty of Darwin's conceptions for as we have seen not only had the idea of organic evolution been from the earliest times amongst the speculative conceptions of Philosophers but it had also been developed in detail especially by Lamarck and Erasmus Darwin. Even the special conception of Natural Selection as a factor in Evolution although it arose quite independently in Darwin's mind had been suggested by several earlier writers as Darwin himself acknowledged. The vast influence of Darwin's work was due in the first place to the fact that he for the first time firmly established the fact of the transmutation of species as an induction resting upon a vast accumulation of facts obtained by observation and experiments of very various kinds. In the second place he for the first time worked out the special theory of Natural Selection as a prime factor in Evolution and adduced a great and varied array of facts illustrating the modes of its operation. The first of these elements in Darwin's work was ultimately decisive in its effects. After a period of bitter strife the fact of organic evolution has been accepted by all Biologists and by the educated part of the general public. After Darwin's work it was no longer a speculative hypothesis but a well attested deduction from observation. As regards the position of Natural Selection as the chief factor in Evolution it is not possible to speak so positively. On this matter the opinions of Biologists have been and still are much divided; a fact to which I shall refer later in some detail.
The arguments employed by Darwin in support of his first main thesis the relation of species to one another by descent are drawn from various departments of Science; and he shows that they all point to the same conclusion. In Darwin's opinion the most conclusive evidence is drawn from the facts disclosed by Embryology. The resemblance between the embryos of various animals is much closer than the resemblance between the adults; the fact that the embryos of vertebrates such as birds and snakes are almost indistinguishable from one another at the earliest stage of their development; the similarity in embryos of homologous parts which become later on differentiated; all these point back to an ancestor common to a whole group of different animals. A strong confirmation is afforded by the survival of vestigial organs. >From the morphological point of view Darwin deduces that unity of type represents actual relationship between the species which possess that unity. From Palaeontology he produced cogent evidence dependent on the close similarity of the fossil remains of two successive geological formations. He shows that more highly organized forms of life have been developed successively and gradually from parents closely resembling them. The facts of geographical distribution of species and fauna form an important part of his argument. The fact that the faunae of regions in which the geographical and climatic conditions are not very dissimilar diverge widely from one another is taken to indicate the local development of those faunae. The various barriers to migration are to be regarded as an important factor; valuable indications being given by an examination of the fauna of islands. Thus when an island is sufficiently far from a continent it can be observed that certain genera of animals which exist on the continent are entirely lacking on the island. It will be observed that the theory of descent or of such a relationship involving the transmutation of species is a descriptive theory of the most general kind; a wide generalization starting from the perceptual fact of the relation between individual animals and plants and their offspring rising to the conception that the existence of the most diverse species of animals and plants can be unified by universalizing this one kind of relationship. It takes the fact of heredity as the fundamental fact but assumes on the basis of induction from a great mass of observed facts that the relation of similarity between parent and offspring is not so close but that it is consistent with extreme dissimilarity between individual organisms which are to be conceived as connected with one another by a complex of relationships (not a single chain) of which the unit is the single relation which holds between parent and offspring. Subject to this restriction the theory is independent of any special conception such as that of germ-plasm as to the details of the relation between parent and offspring; that is as to any mechanism by which this relation of heredity can be described. It is also independent of any theory of the origin nature or magnitude of the variations or dissimilarities between parent and offspring which by their accumulation in the whole complex of which the relation between individuals of different species consists amount to the actual dissimilarity between such individuals.
The second main thesis of Darwin is that the chief factor in Evolution is natural selection. Setting out from the existence of small heritable spontaneous variations in the individual organism that is of small deviations from the average of the stock to which the individual belongs the structural variations of a particular character being in the directions both of excess and defect in accordance with the theory those of the variations which give some advantage to the individual over his fellows in the struggle for existence lead to the preservation of the individuals possessing and propagating them in preference to those who do not possess them. This selective process going on through many generations in which the originally small variations are increased by accumulation at every stage giving an advantage to those individuals in which the variations are in the profitable direction leads ultimately to the survival of a stock which differs from the original stock in respect of the particular characters; thus a new stock more fitted to the environment than was the original stock comes into being. In Darwin's own words: “Natural Selection acts only by the preservation and accumulation of small inherited modifications each profitable to the preserved being.” It should be observed that the doctrine does not include any account of the mode in which the variations arise or are inherited; their existence as variations of small but varying magnitude in every direction and both in excess and defect relatively to the mean and heritable is simply accepted as a datum derived from observation. These variations are frequently described as “fortuitous” or as “spontaneous” which can only be taken to express the fact that they are not assumed to be variations in any particular direction or to be the outcome of any assigned process.
From a general point of view this theory is remarkable on account of its anti-teleological tendency; for it is designed to account for what has prima facie the appearance of a process of progressive purposive adaptation to the environment; it substitutes an account of the mode in which such adaptation can be conceived to arise which does not involve at any stage any element which requires the notion of guidance towards an end due either to conscious or unconscious agency. When such an expression as “struggle for existence” is employed in connection with this theory the expression must be taken in a very broad sense. The struggle may last for many generations and does not necessarily imply violent individual contests in which the less favoured individuals succumb. A slow racial elimination of the less fit owing to their lesser ability than the more fit to cope with their fellows with their racial enemies to obtain sustenance and to propagate their kind must be included in the denotation of the expression. Natural selection provides not only for the progressive adaptability to the environmental conditions but for the transmutations of the adaptations when they have reached a maximum.
It is impossible and indeed for my purpose unnecessary to give any account of the wealth of the evidence which Darwin gave in support of his theory or of the many illustrations and applications of it which he gave. Many objections to it were seen and dealt with by Darwin himself; others have been since advanced in the course of the ceaseless discussion of the factors in evolution which has taken place since the theory was advanced and during which the attention of researchers has been largely devoted to the origin and mode of inheritance of variations which remained entirely open in the original theory. In the course of his work Darwin paid a great deal of attention to the artificial selection of varieties amongst domesticated plants and animals and especially in the breeding of pigeons. In this case the purpose of the gardener or breeder plays an effective part in selecting for propagation those individual plants or animals which “spontaneously” according to a common expression exhibit those variations which the gardener or breeder selects as according with his purpose. This is quite consistent with the anti-teleological character of Darwin's theory for the purposiveness is not immanent in the plants and animals but is in the mind of the gardener or breeder. The selection in the case of undomesticated animals and plants differs from this only in the fact that no gardener or breeder interferes in its free play.
Darwin believed that natural selection is sufficient to account for the evolution of the most complicated organs but he at all times admitted the existence of other contributory factors of evolution especially of what may be described as the Lamarckian factor the dependence on inherited effects of use and disuse. Thus he writes1:
It is generally acknowledged that all organic beings have been formed on two great laws—Unity of Type and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class and which is quite independent of their habits of life. On any theory unity or type is explained by unity of descent. The expression of conditions of existence so often insisted on by the illustrious Cuvier is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during past periods of time; the adaptations being aided in many cases by the increased use or disuse of parts being affected by the direct action of the external conditions of life and subjected in all cases to the several laws of growth and variation. Hence in fact the law of the Conditions of Existence is the higher law; as it includes through the inheritance of former variations and adaptations that of Unity of Type.
It has been however pointed out by E. S. Russell that Cuvier meant by “Conditions of Existence” not environmental conditions as Darwin assumed but the coordination of parts to form the whole of an organism.
It has been held that Darwin gives little weight in his theory to and does not account for the correlation of variations in different parts of the organism. In the course of time Darwin somewhat modified his belief in the relative weight of the factor of natural selection as compared with the factor due to the direct action of the environment. Thus in 1862 in a letter to Lyell he writes: “I hardly know why I am a little sorry but my present work is leading me to believe rather more in the direct action of physical conditions”; again in 1876 he wrote to Moritz Wagner: “When I wrote the Origin and for some years afterwards I could find little good evidence of the direct action of the environment; now there is a large body of evidence.” In view of the great influence which Darwin's theories have exercised upon modern views of mental evolution and upon Sociology it is important to observe that Darwin recognizes psychological factors as contributory to organic evolution. In the Descent of Man he advocated the view “that there is no fundamental difference between man and the higher mammals in their mental faculties.” In this matter he differed from A. R. Wallace who held that the higher mental faculties of man had been derived from a special stream of spiritual influence at some period in the evolution of the race.
Darwin had no theory of mental evolution but took into account the fact that conscious experience with concomitant physiological processes occurs in connection with some kinds of animal behaviour. The close correlation of psychological and physiological processes he accepts without entering upon the difficult and highly controversial philosophical discussions which arise in this connection. If the theory of psycho-physical parallelism be accepted a psychological description can be regarded as a merely convenient form in which the precisely corresponding organic processes in the brain and nervous system can be denoted. There is no evidence that Darwin accepted this theory in its complete sense but he appears to have accepted at least implicitly the methodological hypothesis that mental evolution is correlated with organic evolution through heredity. Only by the assumption of this regulative idea can psychological factors be recognized as within the domain of Natural Science; without it no account of the behaviour of the higher animals would be in any degree adequate which was not based in large part upon a developed mental science as an independent department. The nature of the responses of an organism to stimuli arising from the environment depends upon the detailed structure of the organism; Darwin held that in the main natural selection was the mode in which the organism had been adapted through heredity to respond fittingly to such stimuli. Especially what is known as instinctive behaviour he regarded as the result of racial preparation transmitted through organic heredity principally in accordance with natural selection but in many cases arising partly or wholly from the inheritance of modifications due to use and disuse. But Darwin recognized that instincts can be modified in the course of the individual life. Intelligent modification of behaviour Darwin regarded as due to the combination of incipient variation and acquired modification; under natural selection this combination has survival value. A factor of evolution which Darwin considered to have been of importance in certain cases is sexual selection; and this involves the prima facie recognition of a psychological element since it includes the conception of the choice or preference of the females for males with certain characteristics as the origin of secondary sexual characters other than weapons of offence and defence; these latter being of importance in this connection only in the struggle between males for the possession of the females. Darwin states as regards the female that “it is not probable that she consciously deliberates; but she is most excited or attracted by the most beautiful or melodious or gallant males.” This part of Darwin's theory has been the subject of much criticism on the part of later Naturalists.
The great cleavage of opinion as regards the relative importance in evolution of the two principles that of natural selection and the Lamarckian factor of the inheritance of acquired characters has been represented in the time subsequent to Darwin by the two schools of Neo-Darwinians for whom natural selection is the all-important factor in evolution and the Neo-Lamarckians who regard natural selection as of little importance and lay the chief stress upon the inheritance of acquired characters. This divergence of opinion began in Darwin's own time; A. R. Wallace laying more exclusive stress than did Darwin himself upon Natural Selection and Herbert Spencer being an advocate of the Lamarckian view. A large part of the work of Biologists since the time of Darwin has been devoted to elucidation of the mode in which variations may be conceived to arise in the organism and of the conditions under which modifications of the individual organism become hereditary. The question has also arisen whether the small modifications the existence of which Darwin presupposed as the basis of his theory are as important in relation to evolution as larger variations which arise as discontinuous variations or mutations. The notion of the struggle for existence has been extended to embrace the conception of competition of tissues cells and smaller units within the organism.
The notion that in addition to the personal selection which depends upon the struggle between an individual and the other members of his race and also against foes of different race from himself and against the environment there is also a struggle of parts and cells within the organism was introduced by Roux in 1881. This histonal struggle or competition between the tissues for sustenance may give rise to local modifications which produce acquired adaptations to the environment during the individual lifetime. The question whether such modifications are hereditary and thus whether they have evolutionary value or not falls under the general question of the heritability of acquired characters.
The chief inspiration of the Neo-Darwinian school has been derived from the teaching of Weismann who rejected almost completely the Lamarckian idea of the inheritance of acquired character as a factor in evolution on the grounds that there has been discovered no sufficient proof that such inheritance in fact takes place and that in accordance with his theory of the germ-plasm as the inherited substance such inheritance is on theoretical grounds impossible. I have in the last lecture given a necessarily summary account of the detailed scheme which was gradually developed by Weismann for the purpose of representing or as he would have said of explaining the facts of inheritance. The chief contribution which Weismann has made based on his theory of germ-plasm to evolutionary theory is the introduction of the idea of germinal selection. This consists in an extension of the notion of natural selection to the sphere of the germ-plasm with its representative units of ids determinants and biophors. This extension is employed to supplement and complete Darwin's theory by providing a theory of the mode in which the hereditary variations may be conceived to arise. According to Weismann there is not only the struggle between organs tissues and cells going on in the organism as pointed out by Roux but also a struggle between the determinants. When the determinants multiply by cleavage the new determinants differ in size and in power of assimilation of nutrition. Those determinants which have greater power of assimilation become stronger than the others and increase their superiority over the others in this respect. This struggle is resumed in every successive generation as each generation receives its germ-plasm with its determinants from the preceding one. The result is that gradually the parts of the organism which are represented by the more vigorous determinants which are successful in the struggle become more strongly marked. This is held to explain the accumulation of small modifications in certain specific directions. The explanation of the fact that useful variations are always to be found and are increased is that the corresponding determinants are better nourished and their offspring are stronger than those of other determinants. Whether the original appearance of useful modifications apart from their increase when already present can be thus accounted for has been doubted by critics of the theory. Weismann also regards his theory as explaining the correlation of different parts of the organism in the product of adaptations; and also the degeneracy of useless organs he regards as due to the fact that the determinants corresponding to them obtain less nutriment than the others and have consequently weaker offspring; this process of gradual weakening of the particular kind of determinants leads gradually in the course of many generations to the obliteration of the organ corresponding to them or to its only remaining as a vestige. The fact of the reappearance of ancestral characters is explained by Weismann as due to a struggle between the various ids determinants and biophors in the fertilized egg-cell. Since these are derived from the germ-plasm of both parents and that germ-plasm in each case contains ancestral germ-plasm there is occasion for a contest in which certain selected representative particles become effective in performing the function of forming the embryo whilst others remain only in germ-plasm which is transmitted unchanged to the offspring. Weismann's theory of germinal selection is intended to explain why variations in a fixed direction take place as well as why complex organs with many correlated parts appear whilst these facts remain unexplained by Darwin's theory of selection in its original form. Although Weismann was strongly opposed to the Lamarckian view he has admitted that in some cases nutritive and other environmental conditions may produce heritable modifications by direct action upon the germ-plasm within the body but he regards direct action upon the somatic plasm as quite insufficient to produce such heritable variations.
Weismann's theory is a very complicated one which he gradually evolved in various stages; it has elements in common with other theories but these elements he adapted for his particular purposes. Of all the theories which work with representative bodies in the germ-plasm it is the most complete and it aims whether successfully or not is a matter of controversy at resuming a larger complex of facts than any other such theory. Like every theory dealing with evolution or with vital phenomena in general it ends up at a point in which the real difficulties of explanation of the facts of the existence and activities of living organisms are pushed back and concentrated on certain living elementary beings such as biophors whose existence and activities as living beings are simply postulated without further analysis. This characteristic of such theories is precisely parallel to the analogous case of physical and chemical theories in which the phenomena of inorganic matter are made to depend upon properties assigned to postulated conceptions such as electrons corpuscles or atoms in accordance with certain postulations. In both cases when a theory has reached at least a provisionally definite form the theory does not analyse further the ultimate postulations upon which it rests. Thus in the rigorous sense of the term explanation such a scientific theory is no nearer an explanation of the phenomena than at its starting point; and yet when the necessary character of this restriction as inherent in Natural Science has been grasped it will be seen that this does not detract from the utility of the theory as a descriptive scheme.
During the time which has elapsed since Darwin's investigations led to the complete acceptance by the Scientific world of the fact of evolution of species the scrutiny of the factors of evolution has led to an enormous amount of discussion and of detailed work of observation with a view to its elucidation. A large number of theories have been propounded of which it is impossible for me to give any account. I must confine myself to an indication of the main features of a theory which recognizes the importance of a class of facts which were regarded by Darwin as of little or no importance in relation to evolution. Besides the originally small variations accumulating by slow changes and consequently known as continuous variations the role of which in Darwin's theory was a fundamental one there exists another mode of variation which consists of a sudden or discontinuous variation. The characters of a species or of a variety sometimes undergo a sudden modification not due to the accumulation of continuous variations. These discontinuous variations some cases of which were mentioned by Darwin himself have since been the subject of attention by many naturalists who considered them to be of importance in relation to evolution. A developed theory of evolution regarded as dependent upon these discontinuous variations or mutations has been propounded by the Dutch Botanist De Vries and rests upon the basis of a large number of experiments on transplanted wild plants and on various cultivated plants in a botanical garden at Amsterdam. For the slowly accumulated variations in the Darwinian theory which may require an enormous amount of time to produce such changes as the evolution of species contemplates De Vries proposes to substitute periodical but sudden and quite noticeable steps. He observes that “this assumption only requires a limited number of mutative periods which might well occur within the time allowed by physicists and geologists for the existence of animal and vegetable life on the earth.” This theory of periodical mutations is regarded as consonant not only with the fact that species or varieties change but as reconciling that fact with the constancy of species for long periods of time subject to individual fluctuations:
Mutability (he writes) is not a permanent feature but a periodic phenomenon producing at times new qualities and at other times leaving the plants unchanged during long successions of generations. All lines of the genealogic tree show alternating mutating and constant species. Some lines may be mutating at the present moment; others may momentarily be constant.... In a complete and systematic enumeration of the real units of nature the elementary species and varieties are thus observed to be discontinuous and separated by definite gaps. There is no reason to suppose that the world is reaching the end of its development and so we are to infer that the production of new species and varieties is still going on. In reality new forms are observed to originate from time to time both wild and in cultivation and such facts do not leave any doubt as to their origin from other allied types and according to natural and general laws.
De Vries in formulating his theory of the laws of mutation maintains that new elementary species appear suddenly without intermediate steps springing laterally from the main stem; and several such new species may arise from the parental form at once in accordance with his experiments. The new elementary species attain their full constancy at once and transmit their characters to their progeny independently of any external conditions. Some of the new strains are evidently elementary species while others are to be considered as retrograde varieties. The term species is used by De Vries in a sense not identical with the Linnaean species; his elementary species is more nearly what is denoted by variety in the ordinary classification. De Vries attributes an important part in evolution to natural selection but he regards it as operative between species and not between individuals of the same species. The external environment he regards as influencing this interspecific selection and as also probably determining the appearance of a period of mutation but otherwise of no influence. The origin of the mutations he regards as germinal congenital and depending on changes within the sexual cells.
Many cogent criticisms have been made of the theory of De Vries. The importance of the mutations he describes in evolution in general has been the subject of much controversy but his experiments show clearly that the mutations play some part as a factor in evolution; how great that part is or how general remains for determination in the future.