Hormones and Heredity | Page 6

as the 'Fertilisation of Orchids' or 'Earthworms.'
Other speculations in Biology are related to Taxonomics or Bionomics according as they deal with the structure of the dead organism or the action of the living. Anatomy and its more theoretical interpretation, morphology, are related to Taxonomics, physiology and its branches to Bionomics. In fact, the fundamental principles of physiology must be understood before the study of Bionomics can begin. We must know the essential nature of the process of respiration before we can appreciate the different modes of respiration in a whale and a fish, an aquatic insect and a crustacean. The more we know of the physiology of reproduction, the better we can understand the sexual and parental habits of different kinds of animals.
The two branches of biological study which we are contrasting cannot, however, be completely separated even by those whose studies are most specialised. In Bionomics it is necessary to distinguish the types which are observed, and often even the species, as may be illustrated by the fact that controversies occasionally arise among amateur and even professional fishermen on the question whether dog-fishes are viviparous or oviparous, the fact being that some species are the one and others the other, or the fact that the harmless slow-worm and ring-snake are dreaded and killed in the belief that they are venomous snakes. Taxonomics, on the other hand, must take account of the sex of its specimens, and the changes of structure that an individual undergoes in the course of its life, and of the different types that may be normally produced from the same parents, otherwise absurd errors are perpetrated. The young, the male, and the female of the same species have frequently been described under different names as distinct species or even genera. For example, the larva of marine crabs was formerly described as a distinct genus under the name of Zoaea, and in the earlier part of the nineteenth century a lively controversy on the question was carried on between a retired naval surgeon who hatched Zoaea from the eggs of crabs, and an eminent authority who was Professor at Oxford and a Fellow of the Royal Society, and who maintained that Zoaea was a mature and independent form. In the end taxonomy had to be altered so as to conform with the fact of development, and the name Zoaea disappeared altogether as that of an independent genus, persisting only as a convenient term for an important larval stage in the development of crabs.
These two kinds of study give us a knowledge of the animals now living. But we find it a universal rule that the individual animal is transitory, that the duration of life, though varying from a few weeks to more than a century, is limited, and that new individuals arise by reproduction, and we have no evidence that the series of successive generations has ever been interrupted; that is to say, the series in any given individual or species may come to an end; species may be exterminated, but we know of no instance of individuals coming into existence except by the process of reproduction or generation from pre-existing individuals. Further, we know from the evidence of fossil remains that the animals existing in former periods were very different from those existing now, and that many of the existing forms, such as man, mammals, birds, bony fishes, can only be traced back in the succession of stratified rocks to the later strata or to those about the middle of the series, evidence of their existence in the periods represented by the most ancient strata being entirely absent. Existing types then must have arisen by evolution, by changes occurring in the succession of generations.
These three facts--namely, the limited duration of individual life, the uninterrupted succession of generations, and the differences of the existing animals and plants from those of former geological periods whose remains are preserved in stratified rocks--are sufficient by themselves to prove that evolution has taken place, that the history of organisms has been a process of descent with modification. If the animals and plants whose remains are preserved as fossils, or at any rate forms closely related to these, were not the ancestors of existing forms, there are only two other possibilities: either the existing forms came into existence by new creations after the older forms became extinct, or the ancestors of existing forms, although they coexisted with the older forms, never left any fossil remains. Each of these suppositions is incredible.
In view of these plain facts and their logical conclusion it is curious to notice how Darwin in his Origin of Species constantly mingles together arguments to prove the proposition that evolution has occurred, that the structure and relations of existing animals can only be explained by descent with modification, with arguments