109. Nay, so definite is the course of growth, that when a limb or part of a limb is cut off from a crab or salamander, a new limb or new part is reproduced in the old spot, exactly like the one removed. Bonnet startled the world by the announcement that the _Naïs_, a worm common in ponds, spontaneously divided itself into two worms; and that when he cut it into several pieces, each piece reproduced head and tail, and grew into a perfect worm. This had been accepted by all naturalists without demur, until Dr. Williams, in his “Report on British Annelida, 1851,” declared it to be a fable. In 1858, under the impulse of Dr. Williams’s very emphatic denial, I repeated experiments similar to those of Bonnet, with similar results. I cut two worms in half, and threw away the head-bearing segments, placing the others in two separate vessels, with nothing but water and a little mud, which was first carefully inspected to see that no worm lay concealed therein. In a few days the heads were completely reformed, and I had the pleasure of watching them during their reconstruction. When the worms were quite perfect, I again cut away their heads, and again saw these reformed. This was repeated, till I had seen four heads reproduced; after which the worms succumbed. 110. The question naturally arises, Why does the nutritive fluid furnish only material which is formed into a part like the old one, instead of reproducing another part, or one having a somewhat different structure? The answer to this question is the key to the chief problem of organic life. That a limb _in situ_ should replace its molecular waste by molecules derived from the blood, seems intelligible enough (because we are familiar with it), and may be likened to the formation of crystals in a solution; but how is it that the limb _which is not in existence_ can assimilate materials from the blood? How is it that the blood, which elsewhere in the organism will form other parts, here will only form this particular part? There is, probably, no one who has turned his attention to these subjects who has not paused to consider this mystery. The most accredited answer at present before the world is one so metaphysiological that I should pass it by, were it not intimately allied with that conception of Species, which it is the object of these pages to root out. It is this: