The theory of Evolution then suggests that the same processes which are employed by the cattle-breeder have been in operation through untold ages.  For the intention and care of the human agent, Mr. Darwin substitutes two principles; one designated as “Natural Selection,” the other as “Sexual Selection.”  For their full development he claims unlimited time.  The ground on which the Process of Natural Selection is maintained is as follows:—­

It has been already noticed that no two individuals of the same kind are exactly alike in all respects; each individual has some peculiarities, generally very trifling, but sufficient to distinguish it from all other individuals.  Some of these peculiarities will probably be such as to be of some service to the individual in the struggle of life; they will assist it in procuring food, or in resisting or escaping from its natural enemies, while on the other hand the peculiarities of other individuals will be prejudicial to them in these ways.  The consequence will be that a larger proportion of those having favourable peculiarities will survive and propagate their kind; their offspring will inherit the peculiarities of their parents, and reproduce them in various degrees.  The same process will then be repeated, and thus from generation to generation the peculiarity will be increased, till at last it is sufficient to mark out, first a new variety, then a new species, and so on.  This process then, continued through a long course of ages, was at one time considered by Mr. Darwin sufficient to account for all the varieties of living creatures now existing, or that have existed in past ages.  But he has more recently satisfied himself [Footnote:  Descent of Man, vol. i p. 152.] that there are many phenomena which are not satisfactorily accounted for by this principle, since many of the specific differences of animals are found to exist in matters which, cannot directly promote their success in the struggle of life.  Such, for instance, are the brilliant colours which are found, especially among the males, in many species of birds.  These he proposes to explain by the supplementary theory of “Sexual Selection.”  His suggestion is that these peculiarities are in some way attractive to animals of the opposite sex, so that the individuals in which they are most strongly developed are more successful than others in obtaining mates, and that in this way the peculiarity is gradually fixed and increased.

By these two processes, then, Mr. Darwin supposes that all the differences now existing among animals have been produced and perpetuated; and not only that, but that man also is the result of similar processes, acting through a very long period; that the progeny of certain “anthropomorphous apes” have, by slow degrees, risen in the scale of being above their progenitors; that all our faculties, intellectual and moral as well as physical, differ from those possessed by lower animals in degree only, and not in kind, [Footnote:  Descent of Man, chaps, ii.-v.] so that man has arrived at his present state by what may be termed purely natural processes, without the intervention of any external power.