The sperm and the germ-cells contain the primary elements of all organic structures, and both possess the special qualities and conditions by which they may evolve organic beings. Every cell is composed of minute grains, within which vital action takes place. The interior of a cell consists of growing matter; the exterior, of matter which has assumed its form and is less active. When the vital principle is communicated to it, the cell undergoes a rapid transformation. While this alteration takes place within the cell, deteriorating changes occur in the cell-wall. Although vital operations build up these structures, yet the animal and nervous functions are continually disintegrating, or wasting, them. Throughout the animal kingdom, germ-cells present the same external aspect when carefully examined with the microscope. No difference can be observed between the cells of the flowers of the oak and those of the apple, but the cells of the one always produce oak trees, while those of the other always produce apple trees. The same is true of the germs of animals, there being not the slightest apparent difference. We are unable to perceive how one cell should give origin to a dog, while another exactly like it becomes a man. For aught we know, the ultimate atoms of these cells are identical in physical character; at least we have no means of detecting any difference. SPECIES. The term species is generally used merely as a convenient name to designate certain assemblages of individuals having various striking points of resemblance. Scientific writers, as a rule, no longer hold that what are usually called _species_ are constantly unvarying and unchangeable quantities. Recent researches point to the conclusion that _all species vary more or less_, and, in some instances, that the variation is so great that the limits of general specific distinctness are sometimes exceeded. Our space will not permit us to do more than merely indicate the two great fundamental ideas upon which the leading theories of the time respecting the origin of species are based. These are usually termed the doctrine of _Special Creation_ and the doctrine of _Evolution_. According to the doctrine of Special Creation, it is thought that species are practically immutable productions, each species having a _specific centre_ where it was originally created, and from which it spread over a certain area until its further progress was obstructed by unfavorable conditions. The advocates of the doctrine of Evolution hold, on the contrary, that species are not permanent and immutable, but that they are subject to modification, and that "the existing forms of life are descendants by true generation of pre-existing forms."[1] Most naturalists are now inclined to admit the general truth of the theory of evolution, but they differ widely respecting the mode in which it occurred. THE PROCESS OF GENERATION.