He who wishes to explain Generation must take for his theme the organic body and its constituent parts, and philosophize about them; he must show how these parts originated, and how they came to be in that relation in which they stand to each other. But he who learns to know a thing not only from its phenomena, but also its reasons and causes; and who, therefore, not by the phenomena merely, but by these also, is compelled to say: 'The thing must be so, and it cannot be otherwise; it is necessarily of such a character; it must have such qualities; it is impossible for it to possess others' - understands the thing not only historically but truly philosophically, and he has a philosophic knowledge of it. Our own Theory of Generation is to be such a philosphic comprehension of an organic body, a very different one from one merely historical.