9o THE POPULAR SCIENCE MONTHLY 



the heart begin to beat on the third day of incubation. It all impressed 

 him to an extent that led to a treatise on generation. 



To account for what he saw, he conceived the egg — the female con- 

 tribution — to be essentially passive, containing elements that could be 

 wakened into life by the active principle of the male. This he con- 

 ceived to be a sort of enzyme, a ferment, which acted upon the female 

 germinal substance like rennet upon milk. From this simple beginning 

 he believed the development to progress, organ following organ; and 

 since the spermatic fluid, the active principle, was itself unorganized, he 

 rejected the possibility that parts should preexist. 



Crude as all this is, it was an approximation to the truth, based on 

 the facts as Aristotle had observed them. To this extent, his theory of 

 development has a modern look. On a second glance, however, one dis- 

 covers signs of the same eagerness for final explanations that we have 

 already observed in our discussion of the problem of evolution. How, 

 from so simple a beginning, was the remarkable complexity of the adult 

 structure to be differentiated? And how was the fact to be explained 

 that chick eggs, when they develop, always produce chicks, turtle eggs 

 turtles ; that animals reproduce after their kind ? These were problems 

 that at once engaged his attention, and were answered with character- 

 istic promptness and confidence. Though the germ may be substantially 

 simple, it is subject to two transcendental potentialities that constrain 

 its development with reference to species and form. 



And here Aristotle lapses out of the company of objective scientists. 

 To say that an egg reaches a certain form because it possesses the poten- 

 tiality to reach that form, is like defining a word in terms of itself. It 

 is hardly the type of interpretation to commend itself to modern inves- 

 tigators. Yet it has been the refuge of many minds throughout the 

 ages, and in a more refined and subtle form is used to-day by the dis- 

 tinguished author of " The Science and Philosophy of the Organism," 

 to mask the hopelessness in his retreat from the firing line of experi- 

 mental biology. 



It is the ugly function of final explanations, causes, elements, prin- 

 ciples, in biology, to call a halt. Trust them and, like the genii of old, 

 they whisk one swiftly out of the current of scientific thought. One 

 ceases to ask questions that are amenable to objective tests. And 

 science itself stagnates until such questions germinate again in the 

 minds of men. 



From Aristotle to Caspar Friedrich Wolff extend two thousand years 

 barren of inspiration. Harvey, the famous author of the " Exercitation 

 on the Motion of the Heart and Blood in Animals " ; Malpighi, his great 

 Italian contemporary; and the indefatigable Dutchman, Swammerdam, 

 had each made serviceable observations on the development of mam- 

 mals, birds and insects, but had contributed no new ideas. By the 



