B. THE THEORY OF DESCENT
1. Generalities
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.
I well know that you prefer the name “theory of evolution” for what I am speaking of: but it may be misleading in various respects. We already know that quite a determinate meaning has been given to the word “evolutio” as applied to individual morphogenesis, “evolutio” being here opposed to “epigenesis.” Now there would be nothing against the use of the word evolution in a wider sense—indeed it is often applied nowadays to denote the fact that a something is actually “evolved” in embryology—if only our entelechy had taken the place of the machine of the mechanists. But that is the very point: there must be a real “evolving” of a something, in order that the word evolution may be justified verbally: and that is not the case in so-called phylogeny. At least we know nothing of an evolutionary character in the problematic pedigree of the organisms, as we shall see more fully hereafter. The term “theory of descent” is therefore less open to objection than is the usual English term. The word transformism, as used by the French, would also be a very good title.
The theory of descent is the hypothetic statement that the organisms are really allied by blood among each other, in spite of their diversities.[143] 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 palæontology. 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.