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 contribution—to be essentially passive, containing elements that could be wakened into life by the active principle of the male. This he conceived 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 discovers 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 characteristic 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 potentiality 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 investigators. 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 distinguished author of "The Science and Philosophy of the Organism," to mask the hopelessness in his retreat from the firing line of experimental biology.
It is the ugly function of final explanations, causes, elements, principles, 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 mammals, birds and insects, but had contributed no new ideas. By the
