MODERN SCIENTIFIC THOUGHT 89 



could not have known, in the following passionate challenge of a modern 

 defender of the faith in final causes : 



"Let not science contrive its own destruction by venturing to lay 

 profane hands, vain for explanation, on that sacred human nature which 

 is its very spring and authorizing source." Modern developments in 

 philosophy itself indicate that the challenger need have no fear. What- 

 ever the inevitable expansion of human knowledge may accomplish 

 for human nature will not be by means of violent or profane hands. 

 Conceptions of human nature, like all other conceptions of the human 

 mind, adapt themselves quietly, impersonally, without anguish, to suc- 

 cessive discoveries of truth. 



Ill 



Passing now to the problem of development, one is struck by the 

 modern aspect of Aristotle's contribution. 



Have you ever seen an egg grow? Have you perhaps followed the 

 frog's egg, as it splits up into a group of segments ; seen a cleavage fur- 

 row spread across it, new furrows succeeding each other with every half 

 hour; observed the segments rhythmically swell and flatten with each 

 cleavage; felt the mystery of this marvelous plastic process of develop- 

 ment? Here is life; here is activity. And the juxtaposition of these 

 phrases is not accidental. 



Aristotle knew nothing of the cleavage of the frog's egg. He had no 

 knowledge of the segments thus formed — which are now called cells. 

 He did not know that the egg, is a cell also, comparable with the cells 

 that make up, as fundamental structural units, the various organs and 

 tissues of the body ; that the egg like these other cells, possesses a char- 

 acteristic body called the nucleus, which, as in all nuclei, contains a 

 substance (chromatin) now generally understood to be most intimately 

 concerned with the phenomena of differentiation and heredity. He was 

 ignorant, also, of the nature of the male sex element, vastly smaller than 

 the egg and differing from it remarkably in form, being adapted to a 

 life of great activity. Otherwise, he would have known that the sperm, 

 like the egg, is, in spite of its size and form, a cell, furnished with a 

 nucleus and chromatic substance. And had he lived as late as 1875, he 

 might have known that the essential facts of fertilization consist not 

 only in the stimulation, the activation of the egg by the single sperm 

 which penetrates its substance, but in the fusion of the egg and sperm 

 nuclei and the mixture of the chromatin thus derived from the two 

 sexes. 



Nothing of this Aristotle knew. But he had observed the develop- 

 ment of the chick. Without the microscope he had failed to note the 

 early stages one sees so readily in the frog. But he had seen the embryo 

 gradually appear on the upper side of the inert yolk, and he had seen 



