We have thus far spoken only of the various forms of one factor, viz., the Darwinian factor of selection, whether natural or artificial. We have dwelt upon this one, because the natural and the artificial processes are so similar, and the artificial is so controllable. But there are other factors in operation, in art as well as in nature. We have already spoken (p. 73) of other factors of natural change. We have shown how changing physical environment affects _function_, and function affects _form and structure_, and how these slight changes are integrated by heredity through many generations. We have also shown how _use_ or _disuse_ increases or diminishes the size and change the form of parts, and these changes, also, however slight, are integrated by heredity. Now, these factors are operative also in domestication of animals and cultivation of plants. No environment is so new and peculiar as domestication and cultivation. The soil and temperature in plants, food and housing of domesticated animals, tend to change form and structure of the offspring, although in a way which it is difficult intelligently to control, and thus are prolific of varieties from which to select. In fact, they often give rise to great and unexpected modifications, called sports, which form points of departure for new varieties and races. Now, in nature, not only are all these causes and factors of change in constant operation, but they act together in a peculiarly complex way. All the members of a fauna and flora, and the physical environment of any locality, constitute together a most complex and delicately adjusted system of correlated parts. A change in one part is propagated through the whole system; also, a change in one factor affects all other factors. When we add to this the large amount of time, in comparison with individual human life and observation, necessary to produce visible change of form, we can easily understand why the process is still imperfectly understood, although the _fact_ is certain. But it will be asked, Are there, then, no differences between the artificially made extreme varieties equivalent, so far as difference of form is concerned, to species, and real natural species? There are. If there were not, there would never have been any doubt about the derivative origin of natural species. But if it be asked, Are not these differences fundamental, and therefore fatal to the argument for evolution derived from this source? we answer, we think not. We will deal frankly and fairly with these differences.