On this view all variation arises within the germinal substance, but it is manifested in the body which is its product. How variations arise we do not know with any exactness of detail. That the germinal substance is influenced in its nutrition and in other ways by the surrounding tissues is highly probable; and this influence may lead to changes which are the source of variations; but it is very doubtful whether such influence can be what we before termed “homœopathic.”[202] It is improbable that the formation of the nerve-connections involved in intelligent behaviour which has grown habitual through repetition, can so influence the germinal cells as to give rise to variations of like nature. In other words, acquired habit is probably not a direct determinant of an inherited variation of like nature in instinctive behaviour. Apart from such influence the only source of variations which can be assigned is either the differential division of nuclei in preparation for the process of fertilization,[203] or the process of fertilization itself. The union of perhaps differentiated germinal substance from two distinct parents affords the opportunities for the admixture and compounding of hereditary qualities in the two samples, from which variations favourable or the reverse may arise. It is now generally recognized, however, that the origin of variations is a problem quite distinct from that of the survival of those whose direction is favourable to that end. The theory of natural selection, as such, does not pretend to offer any explanation of the manner in which variations arise; though of course a complete theory of organic evolution must assign the antecedents and conditions of organic progress in all its varied phases. We know that variations do occur; we know, too, that more individuals are born than survive to procreate their kind; and, on the theory of natural selection, we draw from these data the conclusion that, on the average, the animals that escape elimination are those in which the variations are of such a nature as to conduce to this end.