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, 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 COVERT PRESUMPTION OF ALL THEORIES OF DESCENT
We have used very neutral and somewhat figurative words, in order to show what might be called the logical value of the theory of descent, in order to signify its value with respect to so-called “explanation.” We have spoken of the “light” or the “elucidation” which it brings, of the “peculiarity of aspect” which is destroyed by it. We have used this terminology intentionally, for it is very important to understand that a specific though hidden addition is made almost unconsciously to the mere statement of the hypothesis of descent as such, whenever this hypothesis is advocated in order to bring light or elucidation into any field of systematic facts. And this additional hypothesis indeed must be made from the very beginning, quite irrespective of the more detailed problems of the law of transformism, in order that any sort of so-called explanation by means of the theory of descent may be possible at all. Whenever the theory that, in spite of their diversities, the organisms are related by blood, is to be really useful for explanation, it must necessarily be assumed in every case that the steps of change, which have led the specific form A to become the specific form B, have been such as only to change in part that original form A. That is to say: the similarities between A and B must never have become overshadowed by their diversities.
Only on this assumption, which indeed is a newly formed additional subsidiary hypothesis, joined to the original hypothesis of descent in general—a hypothesis regarding the very nature of transformism—only on this almost hidden assumption is it possible to speak of any sort of “explanation” which might be offered by the theory of transformism to the facts of geography, geology, and biological systematics. Later on we shall study more deeply the logical nature of this “explanation”; at present it must be enough to understand this term in its quasi-popular meaning.
What is explained by the hypothesis of descent—including the additional hypothesis, that there always is a prevalence of the similarities during transformism—is the fact that in palaeontology, in the groups of island and continent faunae and florae taken as a whole, as well as in the single categories of the system, the similarities exceed the diversities. The similarities now are “explained”; that is to say, they are understood as resting on but one principle: the similarities are understood as being due to inheritance;[144] and now we have but one problem instead of an indefinite number. For this reason Wigand granted that the theory of descent affords what he calls a numerical reduction of problems.
Understanding then what is explained by the theory of descent with its necessary appendix, we also understand at once what is not elucidated by it: the diversities of the organism remain as unintelligible as they always were, even if we know that inheritance is responsible for what is similar or equal. Now there can be no doubt that the diversities are the more important point in systematics; if there were only similarities there would be no problem of systematics, for there would be no system. Let us be glad that there are similarities in the diversities, and that these similarities have been explained in some way; but let us never forget what is still awaiting its explanation. Unfortunately it has been forgotten far too often.
THE SMALL VALUE OF PURE PHYLOGENY
And so we are led to the negative side of the theory of transformism, after having discussed its positive half. The theory of descent as such, without a real knowledge of the factors which are concerned in transformism, or of the law of transformism, in other terms, leaves the problem of systematics practically where it was, and adds really nothing to its solution. That may seem very deplorable, but it is true.
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 so-called 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, especially if taken together with the few facts known about mutations. 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.