air without into the sterilized liquid, no living animals will ever appear in it. We know of no instance of the spontaneous generation of animals, and all the animals whose life-history we know are produced by other animals of the same kind. =Simplest multiplication and development.=--The simplest method of multiplication and the simplest kind of development shown among animals are exhibited by such simple animals as _Amœba_ and _Paramœcium_. The production of new individuals is accomplished in _Amœba_ by a simple division or fission of its body (a single cell) into two practically equivalent parts. An _Amœba_ which has grown for some time contracts all of its finger-like processes, the pseudopodia, and its body becomes constricted. This constriction or fissure increases inwards so that the body is soon divided fairly in two. There are now two _Amœbæ_, each half the size of the original one; each, indeed, actually one-half of the original one. The original _Amœba_ was the parent; the two halves of it are the young. Each of the young possesses all of the characteristics and powers of the parent; each can move, eat, feel, grow, and reproduce by fission. The only change necessary for the young or new _Amœba_ to become like its parent, is that of simple growth to a size about twice its present size. The development here is reduced to a minimum. Just as the simplest animals perform the other life-processes, such as taking and digesting food, breathing and feeling, in an extremely primitive simple way, so do they perform the necessary life-process of reproduction or multiplication in the simplest way shown among animals.