evolution has taken place is the only thing that can indicate the creative value of the accompanying conditions; whether or no, or to what extent, evolution has been the product of external conditions, of natural selection, or of the fortuitous shuffling of "hereditary units"; or whether, after giving due credit to these agents, we must go still deeper and look for the primary creative factors in a universal, persistent power of growth, and in an internal, automatic adjustment of part to part and part to the whole, which go their ways in spite of the fluctuating influences of heredity, selection and external environment, moving with an increasing momentum of their own along definite paths toward definite ends that are predetermined solely by the nature of organizable materials.
In my judgment, the answer to these problems must come, if at all, from morphology, treated as the formal expression of the dynamics of organic growth, and from the history of its progress as portrayed by the embryology of the individual and the phylogeny of the race. The answer should tell us whether biology shall serve merely to record the phenomena of life, or whether, within its own sphere, it may reasonably hope to attain in some measure the dignity of prophet and master.
II. Missing Links in the Genealogy of the Animal Kingdom
To the layman, the most serious defect in our phylogenetic record is the absence of a connecting link between man and the apes. To the morphologist, dealing with broader aspects of the problem, it is the absence of a whole series of connecting links between the vertebrates and the invertebrates.
The evolution of the vertebrates has extended over many millions of years, from at least the beginning of the Devonian period to the present moment; but during all that time no change in the general plan of their structure has taken place. The vertebrates form an essentially continuous, united group, for the differences between the most widely separated members, as, for example, a fish and a human being, are differences in degree, not in kind; differences in the details of structure, and in the relative location and size of organs and parts of organs, or in the measure of their functions; none whatever in their serial location, in their fundamental structure, or in their mode of growth. Every important part, for example, of the digestive, excretory and reproductive systems, and of the skull, nose, eye, ear, heart and brain of a fish is readily recognized by the trained anatomist in the corresponding organs of man.
It is this broad uniformity in fundamental structure, varied by a continuous series of transitions in organic details, and the historic record of their progress by paleontology, that is the chief measure of blood relationship and community of descent.
