former lecture, that at first the copulating cells were exactly alike in appearance and size, that then one kind of cell became rather larger than the other, and that only gametes which were thus different in size were mutually attractive--the micro-gametes and the macro-gametes, or male and female germ-cells; we have seen that these differences between the two became more and more accentuated, that the female cell continued to grow larger than the male, and to accumulate more and more nutritive material for the building up of the young organism which arises from its union with the male cell, and that the male cells became smaller, but more numerous, as was essential if their chances of finding the often remote female cell were not to disappear altogether. And besides, there are all the innumerable adaptations of the egg-cell to the countless special circumstances which obtain in the different groups, and the innumerable varieties in the form of the sperm-cell, with all its delicate and complicated adaptations to the special conditions under which the egg-cell can be reached and fertilized in this or that group of organisms. Of a truth, he is past helping who does not regard with wonder and admiration the adaptations which have been worked out in this connexion in the course of evolution! But if all these details are adaptations, so is the beginning of the whole process of differentiation; allogamy, the attraction of conjugating cells of different lineage, is not a primary outcome of individual diversity; gametes of different descent did not strongly attract each other of themselves, but they were equipped with a strong power of mutual attraction, because the union of very different individualities was the more advantageous. This is an important distinction, for the adaptation to allogamy is widely distributed, and its latest manifestations have frequently been misunderstood in the same way as its beginnings. The widespread occurrence of allogamy has been interpreted as evidence in favour of the rejuvenation theory, and the endeavour on Nature's part to secure the union of the unlike has been associated with the hypothetical 'rejuvenating' power of amphimixis, and regarded as a direct and inevitable outcome of this. That this view is erroneous we shall see even more clearly from what follows.