Having meditated on the activity of Nature to produce organized bodies, and seen that her power, in this respect, is not limited; having proved that infinity of organic living particles, which constitute life, must exist; having shewn that the living body costs the least trouble to Nature, I now search after the principal causes of death and destruction, and I find that bodies in general, which have the power of converting matter into their proper substance, and to assimilate the parts of other bodies, are the greatest destroyers. Fire, for example, turns into its own substance almost every species of matter, and is the greatest means of destruction known to us. Animals seem to participate of the qualities of flame; their internal heat is a kind of fire; therefore, after fire, animals are the greatest destroyers, and they assimilate and convert into their own substance every matter which may serve them for food: but although these two causes of destruction are very considerable, and their effects perpetually incline to the annihilation of organized beings, the cause of reproduction is infinitely more powerful and active; she seems to borrow, even from destruction itself, means to multiply, since assimilation, which is one cause of death, is at the same time a necessary means of producing life. To destroy an organized being is, as we have observed, only to separate the organic particles of which it is composed; these particles remain separated till they are re-united by some active power. But what is this active power?--It is the power which animals and vegetables have to assimilate the matter that serves them for food; and is not this the same, or at least has it not great connection with that which is the cause of reproduction? CHAPTER III. OF NUTRITION AND GROWTH. The body of an animal is a kind of internal mould, in which the nutritive matter assimilates itself with the whole in such a manner that, without changing the order and proportion of the parts, each receives an augmentation, and it is this augmentation of bulk which some have called _expansion_, because they imagined every difficulty would be removed by the supposition that the animal was completely formed in the embryo, and that it would be easy to conceive that its parts would expand, or unfold in proportion as it would increase by the addition of accessory matter.