The Theory of Descent itself and the differences that obtain even among its adherents can best be studied by considering for a little the works of Reinke and of Hamann.
Reinke, Professor of Botany in Kiel, has set forth his views in his book, “Die Welt als Tat,”(25) and more recently in his “Einleitung in die theoretische Biologie” (1901). Both books are addressed to a wide circle of readers. Reinke and Hamann both revive some of the arguments and opinions set forth in the early days of Darwinism by Wigand,(26) an author whose works are gradually gaining increased appreciation.
It is Reinke’s “unalterable conviction” that organisms have evolved, and that they have done so after the manner of fan-shaped genealogical trees. The Theory of Descent is to him an axiom of modern biology, though as a matter of fact the circumstantial evidence in favour of it is extremely fragmentary. The main arguments in favour of it appear to him to be the general ones; the homologies and analogies revealed by comparative morphology and physiology, the ascending series in the palæontological record, vestigial organs, parasitic degeneration, the origin of those vital associations which we call consortism and symbiosis. These he illustrates mainly by examples from his own special domain and personal observation.
The simplest unicellular forms of life are to be thought of as at the beginning of evolution; and, since mechanical causes cannot explain their ascent, it must be assumed that they have an inherent “phylogenetic potential of development,” which, working epigenetically, results in ascending evolution. He leaves us to choose between monophyletic and polyphyletic evolution, but himself inclines towards the latter, associating with it a rehabilitation of Wigand’s theory of the primitive cells. If, in the beginning, primitive forms of life arose (probably as unicellulars) from the not-living, it is not obvious why we need think of only one so arising, and, if many did so, why they should not have inherent differences which would at once result in typically different evolutionary series and groups of forms. But evolution does not go on _ad libitum_ or _ad infinitum_, for the capacity for differentiation and transformation gradually diminishes. The organisation passes from a labile state of equilibrium to an increasingly stable state, and at many points it may reach a terminus where it comes to a standstill. Man, the dog, the horse, the cereals, and fruit trees appear to Reinke to have reached their goal. The preliminary stages he calls “Phylembryos,” because they bear to the possible outcome of their evolution the same relation that the embryo does to the perfect individual. Thus, _Phenacodus_ may be regarded as the Phylembryo of the modern horse. It is quite conceivable that each of our modern species may have had an independent series of Phylembryos reaching back to the primitive cells. But the palæontological record, and especially its synthetic types, lead Reinke rather to assume that instead of innumerable series, there have been branching genealogical trees, not one, however, but several.