The final result is thus rendered infinitely complex. In some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds. When several breeds have once been formed in any country, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed. Over all these causes of change, the accumulative action of selection, whether applied methodically and quickly, or unconsciously and slowly, but more efficiently, seems to have been the predominant power. _III.--Variation Under Nature_ Before applying these principles to organic beings in a state of nature, we must ascertain whether these latter are subject to any variation. We find variation everywhere. Individual differences, though of small interest to the systematist, are of the highest importance for us, for they are often inherited; and they thus afford materials for natural selection to act and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated productions. Further, what we call varieties cannot really be distinguished from species in the long run, a fact which we can clearly understand if species once existed as varieties, and thus originated. But the facts are utterly inexplicable if species are independent creations. How have all the exquisite adaptations of one part of the body to another part, and to the conditions of life, and of one organic being to another being, been perfected? For everywhere we find these beautiful adaptations. The answer is to be found in the struggle for life. Owing to this struggle, variations, however slight, and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. But the expression, often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate. We have seen that man, by selection, can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations given to him by the hand of Nature. Natural Selection is a power incessantly ready for action, and is as immeasurably superior to man's feeble efforts as the works of Nature are to those of Art.