It is most generally conceded at the present time that the actually existing state of all organisms whatsoever is the result of their history. What does that mean? What are the foundations upon which the assumption rests? What is the relation of systematics to history? In raising such questions and considerations we are treading the ground sacred to the theory of descent.1
The theory of descent is the hypothetic statement that the organisms are really allied by blood among each other, in spite of their diversities. The question about their so-called monophyletic or polyphyletic origin is of secondary importance compared with the statement of relationship in general.
There are two different groups of facts which have suggested the idea of transformism: none of these facts can be said to be conclusive, but there certainly is a great amount of probability in the whole if taken together.
The first group of evidences which lead to the hypothesis of the real relationship of organisms consists of facts relating to the geographical distribution of animals and plants and to palaeontology. As to geography, it seems to me that the results of the floral and faunal study of groups of islands are to be mentioned in the first place. If, indeed, on each of the different islands, A B C and D, forming a group, the species of a certain genus of animals or plants are different in a certain respect, and show differences also compared with the species living on the neighbouring continent, of which there is geological evidence that the islands once formed a part, whilst there is no change in the species on the continent itself for very wide areas, then, no doubt, the hypothesis that all these differing species once had a common origin, the hypothesis that there is a certain community among them all, will serve to elucidate in some way what would seem to be very abstruse without it. And the same is true of the facts of palaeontology. In the geological strata, forming a continuous series, you find a set of animals, always typical and specific for every single stratigraphical horizon, but forming a series just as do those horizons. Would not the whole aspect of these facts lose very much of its peculiarity if you were to introduce the hypothesis that the animals changed with the strata? The continuity of life, at least, would be guaranteed by such an assumption.
The geographical and geological evidences in favour of the theory of descent are facts taken from sciences which are not biology proper; they are not facts of the living, but only facts about the living. That is not quite without logical importance, for it shows that not biology alone has led to the transformism hypothesis. Were it otherwise, transformism might be said to be a mere hypothesis ad hoc; but now this proves to be not the case, though we are far from pretending that transformism might be regarded as resting upon a real causa vera.
But let us study the second group of facts which support the theory of descent. It is a group of evidences supplied by biology itself that we meet here, there being indeed some features in biology which can be said to gain some light, some sort of elucidation, if the theory of descent is accepted. Of course, these facts can only be such as relate to specific diversities, and indeed are facts of systematics; in other words, there exists something in the very nature of the system of organisms that renders transformism probable. The system of animals and plants is based upon a principle which might be called the principle of similarities and diversities by gradation; its categories are not uniform but different in degree and importance, and there are different kinds of such differences. No doubt, some light would be shed upon this character of the system, as revealed by comparative anatomy and embryology, if we were allowed to assume that the relation between similarities and diversities, which is gradual, corresponded to a blood-relationship which is gradual also.
The theory of descent as such, however, without a real knowledge of the dynamic factors which are concerned in transformism, or of the law of transformism, does not yet tell us very much.
Imagine so-called historical geology, without any knowledge of the physical and chemical factors which are concerned in it: what would you have except a series of facts absolutely unintelligible to you? Or suppose that some one stated the cosmogenetic theory of Kant and Laplace without there being any science of mechanics: what would the theory mean to you? Or suppose that the whole history of mankind was revealed to you, but that you had absolutely no knowledge of psychology: what would you have but facts and facts and facts again, with not a morsel of real explanation?
But such is the condition in which pure phylogeny stands. If it is based only on the pure theory of transformism, there is nothing explained at all. It was for this reason that the philosopher Liebmann complained of phylogeny that it furnishes nothing but a “gallery of ancestors”. And this gallery of ancestors set up in phylogeny is not even certain; on the contrary, it is absolutely uncertain, and very far from being a fact. For there is no sound and rational principle underlying phylogeny; there is mere fantastic speculation. How could it be otherwise where all is based upon suppositions which themselves have no leading principle at present? I should not like to be misunderstood in my polemics against phylogeny. I fully grant you that it may be possible in a few cases to find out the phylogenetic history of smaller groups with some probability, if there is some palaeontological evidence in support of pure comparative anatomy; and I also do not hesitate to allow that such a statement would be of a certain value with regard to a future discovery of the “laws” of descent. But it is quite another thing with phylogeny on the larger scale. Far more eloquent than any amount of polemics is the fact that vertebrates, for instance, have already been “proved” to be descended from, firstly, the amphioxus; secondly, the annelids; thirdly, the Sagitta type of worms; fourthly, from spiders; fifthly, from Limulus, a group of crayfishes; and sixthly, from echinoderm larvae. That is the extent of my acquaintance with the literature, with which I do not pretend to be specially familiar. Emil du Bois-Reymond said once that phylogeny of this sort is of about as much scientific value as are the pedigrees of the heroes of Homer, and I think we may fully endorse his opinion on this point. We must, therefore, search for the dynamic factors which promote phylogeny.
HISTORY AND SYSTEMATICS
But first a few words should be devoted to the relations between history and systematics in biology. Is there no contradiction between historical development and a true and rational system which, we conceded, might exist some day in biological sciences, even though it does not at present? By no means. A totality of diversities is regarded from quite different points of view if taken as the material of a system, and if considered as realised in time. We have said that chemistry has come very near to proper rational systematics; but the compounds it deals with at the same time may be said to have originated historically also, though not, of course, by a process of propagation. It is evident at once that the geological conditions of very early times prohibited the existence of certain chemical compounds, both organic and inorganic, which are known at present. None the less these compounds occupy their proper place in the system. And there may be many substances theoretically known to chemical systematics which have never yet been produced, on account of the impossibility of arranging for their proper conditions of appearance, and nevertheless they must be said to “exist”. “Existence”, as understood in systematics, is independent of special space and of special time, as is the existence of the laws of nature: we may speak of a Platonic kind of existence here. Of course it does not contradict this sort of ideal existence if reality proper is added to it.
Thus the problem of systematics remains, no matter whether the theory of descent be right or wrong. There always remains the question about the totality of diversities in life: whether it may be understood by a general principle, and of what kind that principle would be. As, in fact, it is most probably by history, by descent, that organic systematics is brought about, it of course most probably will happen some day that the analysis of the causal factors concerned in the history will serve to discover the principle of systematics also.
The Prerequisites of the Theory of Descent and our Knowledge about them
The theory of descent, of whatever form it may be, has two absolutely indispensable prerequisites: there must exist some sort of deviation from inheritance in some cases, and the result of this deviation must be inheritable itself. Otherwise no phylogeny would be possible.
Now deviations from inheritance are known in four different forms. Let us enumerate them, and let us try to find out whether their results are themselves inheritable or not.
There is, first, continuous variation in the proper sense. All the individuals of the offspring of a pair of animals (or of a parthenogenetic female) are different from each other. And they are different—what is important to notice—even if mother and father belong to the same pure line (Johannsen), i.e. if they are not “mutants” with regard to one another in even the slightest way. Continuous variations follow the “law of errors” as mathematically formulated by Gauss and Galton. Their results are not inheritable, and therefore have no bearing upon the theory of descent. Even if you select two extremes for propagation, you get an offspring with the same error curve. These variations are probably due to the contingent variation of the medium, which also obeys the law of errors.
Secondly, there is adaptive variation, studied in a former part of this book. This is not exactly a “deviation” from inheritance. For the totality of adaptive potencies of a given species is inherited, and it simply depends on the conditions, met by an individual, which potency will become effective. (This point of view, by the way, may also be applied to variation proper.) Nothing is known with certainty about the inheritance of adaptations, i.e. about the so-called “inheritance of acquired characters”, with the exception, perhaps, that the effect of a change of conditions in some (botanical) cases may require several generations before it becomes quite definite. Guyer’s experiments on rabbits, it is true, may mark the beginning of a positive knowledge in this field, but they are rather fragmentary at present and not quite undisputed. About Kammerer’s experiments nothing can be said, until they have been confirmed by a competent author. Inheritance of adaptations, by the way, would not be a very useful event for the organism, unless it would serve to strengthen adaptive results under permanently equal conditions. It might even be said to be in opposition to the real value of adaptation itself.
In the third place there is Mendelian variation. As the single properties of an organism split and combine separately, we may raise new “races” by Mendelian experiments. We may start from a plant with white flowers covered with hairs and from another with red naked ones, and, at the end, we may have before us red flowers covered with hairs and white naked ones. These results, originated as they are at the very basis of all inheritance, are, of course, inheritable themselves. But they do not help us much for phylogeny. For only a regrouping of pre-existing properties, nothing “new”, stands in question.
Ultimately there is mutation (de Vries), which marks a real break of inheritance. Its results are inheritable. This is so-called “discontinuous variation”. Its steps may be comparatively great or very small (Jennings). They are never of such importance as to lead to the formation of a really new “species”. The origin of mutations is unknown. They seem to be due to “inner forces”; but the medium, though in a very unspecified way, may give a sort of stimulus to those forces. Very rich feeding, e.g., has often had a considerable number of mutations as its consequence. No plan or law in the sequence of mutations has so far been discovered. This is all we know, in a positive way, about the “prerequisites” of phylogeny. In fact, it is not very much. And, thus, it is not very surprising that scientists have tried to attack the problem of descent in a merely constructive, hypothetical way. Let us then study the principal forms of this sort of theory.
I do not say “evolution”, as I shall use this word in quite a particular sense later on, and as we do not know a priori whether phylogeny is a true “evolution” or not.