We now approach the _arcana_ of Evolution. The processes we have to deal with in this chapter are not, and probably never will be, the subjects of direct observation. All we can hope to do is to generalize from the results which have risen to the surface of life about the unseen forces from which they spring. The problem is to find (if possible) a generalization which will cover all the facts relating to that modification of natural forms, habits, and instincts which, when it reaches a certain point, means the establishment of a new species. We know that the thing happens, but we shall not understand _how_ it happens until either the mechanism of the process is laid bare, or until it is clear that we are in presence of an agency not entirely definable in terms of mechanical action. The fixity of species is maintained by a number of conditions, chief among which must be reckoned the law of reproduction by conjugation, with the consequent intermixture of numerous different lines of descent. From one point of view conjugation, as Weismann so often insists, greatly favours the adaptability of the organism to new and varied conditions of life, inasmuch as it results in the mingling together in each individual of a great number of varied determinants. But when the conditions are constant, conjugation has also the obvious effect of constantly reabsorbing, as it were, any heritable abnormalities which may occur in individuals or the species, and bringing them back to type. An individual possessing some abnormality of structure will be most unlikely to find a mate possessing the same abnormality—the mate will be either an ordinary individual or will possess, if any, some quite different variation. Their descendants will, therefore, usually show more resemblance to the normal type than to the one abnormal parent, and in their descendants again, for the same reason, the abnormal feature will be still further reduced, until finally it disappears. It is only by the careful selection of mates extending over many generations that pigeon-fanciers, to take one prominent instance, are able to establish a new type. Left to mate uncontrolled among themselves we should never have had the great variety of breeds which have been produced by the art of the fancier from the original rock-pigeon. The small variations which form the starting points of his operations would, under natural conditions, have soon been resolved into the normal type. What is it in nature, then, that sometimes appears to play the part of the intelligent breeder and to urge the plastic forms of life into new moulds?