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. Whatever 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 successive discoveries of truth.
III
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 furrow 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 development? 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 characteristic 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 development 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
