On the whole the belief, which has long been held by many skilful cultivators, that good follows from exchanging seed, tubers, &c., seems to be fairly well founded. Considering the small size of most seeds, it seems hardly credible that the advantage thus derived can be due to the seeds obtaining in one soil some chemical element deficient in the other soil. As plants after once germinating naturally become fixed to the same spot, it might have been anticipated that they would show the good effects of a change more plainly than animals, which continually wander about; and this apparently is the {148} case. Life depending on, or consisting in, an incessant play of the most complex forces, it would appear that their action is in some way stimulated by slight changes in the circumstances to which each organism is exposed. All forces throughout nature, as Mr. Herbert Spencer[330] remarks, tend towards an equilibrium, and for the life of each being it is necessary that this tendency should be checked. If these views and the foregoing facts can be trusted, they probably throw light, on the one hand, on the good effects of crossing the breed, for the germ will be thus slightly modified or acted on by new forces; and on the other hand, on the evil effects of close interbreeding prolonged during many generations, during which the germ will be acted on by a male having almost identically the same constitution. _Sterility from changed Conditions of Life._ I will now attempt to show that animals and plants, when removed from their natural conditions, are often rendered in some degree infertile or completely barren; and this occurs even when the conditions have not been greatly changed. This conclusion is not necessarily opposed to that at which we have just arrived, namely, that lesser changes of other kinds are advantageous to organic beings. Our present subject is of some importance, from having an intimate connexion with the causes of variability. Indirectly it perhaps bears on the sterility of species when crossed: for as, on the one hand, slight changes in the conditions of life are favourable to plants and animals, and the crossing of varieties adds to the size, vigour, and fertility of their offspring; so, on the other hand, certain other changes in the conditions of life cause sterility; and as this likewise ensues from crossing much-modified forms or species, we have a parallel and double series of facts, which apparently stand in close relation to each other.