But we must bethink ourselves: this cannot be the whole truth. The sexual instincts, for which the theory of the neuroses claims a position apart, lead us to quite another point of view. Not all organisms have yielded to the external compulsion driving them to an ever further development. Many have succeeded in maintaining themselves on their low level up to the present time: there are in existence to-day, if not all, at all events many forms of life that must resemble the primitive stages of the higher animals and plants. And, similarly, not all the elementary organisms that make up the complicated body of a higher form of life take part in the whole path of evolution to the natural end, i.e. death. Some among them, the reproductive cells, probably retain the original structure of the living substance and, after a given time, detach themselves from the parent organism, charged as they are with all the inherited and newly acquired instinctive dispositions. Possibly it is just those two features that make their independent existence possible. If brought under favourable conditions they begin to develop, that is, to repeat the same cycle to which they owe their origin, the end being that again one portion of the substance carries through its development to a finish, while another part, as a new germinal core, again harks back to the beginning of the development. Thus these reproductive cells operate against the death of the living substance and are able to win for it what must seem to us to be potential immortality, although perhaps it only means a lengthening of the path to death. Of the highest significance is the fact that the reproductive cell is fortified for this function, or only becomes capable of it, by the mingling with another like it and yet different from it.