334 COMPARATIVE ANATOMY 



Certain it is that nothing fuller of promise has come into biolog- 

 ical science for many years than the set of tendencies in ideas and 

 methods of investigation that have crystallized into the expression 

 " developmental mechanics." It has sometimes seemed to me, 

 though, that the alternative term, causal morphology, would have 

 been more fortunate. The kernel of the thing is, as I understand it, 

 search after the causes of the form of organisms; or really causes of 

 morphogenesis. 



Now where does anatomy come in here? Why, in the first place, 

 it is anatomy, is n't it, that shows us what it is we are seeking the 

 cause of? A rather important preliminary to explaining a thing is 

 to know what we are going to explain. Anatomy, then, in the first 

 place, is the source of supply, so to speak, of the very problems 

 developmental mechanics proposes to solve. But has anatomy 

 exhausted its usefulness in this direction when it has handed out 

 the raw material of a lot of problems? By no means. The moment 

 anatomy becomes seriously comparative, that moment it is on the 

 threshold of causal morphology; for as soon as parts obviously 

 homogeneous, but with considerable differences, are recognized in 

 different groups of animals, a differential in the producing cause of 

 the common part must almost of necessity be assumed by the 

 observer; and since this differential will usually be closer at hand, 

 so to speak, than the cause itself, it is pretty sure to stimulate 

 inquiry as to what the cause has been. And even this much, general 

 and vague though the effort may be, is undoubtedly on the high- 

 road of search after causes of animal form. To illustrate: One's 

 knowledge of the anatomy of the human limbs, let us say, may be 

 complete; but the causes that have produced these limbs are so 

 complicated and obscure, that even were he, as a human anatomist 

 pure and simple, to raise the question of how they came to take 

 the form they have, the absence of even a starting-point for an 

 answer would be apt to prevent an effort in this direction. But now 

 let this anatomist add to his knowledge an acquaintance with 

 the structure of the limbs of, say, a spider monkey. He could 

 hardly escape recognizing that the difference between the limbs 

 of the two animals is connected causally with the different uses to 

 which they are put by their respective possessors. In other words, 

 the cause of the difference in limb structure, being relatively near 

 by and simple, can hardly escape him. He is forced, almost, by 

 comparative anatomy, into the way of causal morphology, or 

 developmental mechanics. But we must recognize that obser- 

 vation, however extensive and painstaking, and however faithfully 

 and thoroughly it be coupled with comparison, and with reflection 

 on the causal efficiency of function, of correlation, of conditions 

 of development, etc., must always still fall short of direct proof of 



