4i 8 THE POPULAR SCIENCE MONTHLY 



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 selec- 

 tion, 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 influ- 

 ences 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. 



