Assuredly the whole intricate complex of adaptations which is now exhibited on the conjugation of the two sex-cells in animals and plants, the differentiation of two kinds of 'sexually' antagonistic cells, with all their special adaptations, the reduction of the chromosomes, the institution of the karyo-kinetic apparatus, together with the centrospheres and so on, cannot possibly have arisen all at once by fortuitous variation, but can only have arisen gradually, step by step, and as the result of 'innumerable external and internal influences.' But why should not these arrangements, nowadays so complex, have had a simple beginning? Why might not this beginning have been the simple union of the protoplasmic bodies of two non-nucleated Monera; followed, after the origin of nuclear substances, by the union of these, and, finally, after the differentiation of a nucleus with a definite number of chromosomes, with a dividing apparatus, with a membrane, and so on, by complete amphimixis as we now know it? And how many transition stages may not be added to fill up the gaps between these three main stages? But how much we can actually prove in regard to these conceivable preliminary stages of amphimixis is another matter. If we take a survey of the observations that have been made up till now, we are confronted at first by the undoubtedly striking fact that very little is known about it as yet, for in fact the whole process is gone through even in quite lowly forms of life in a manner very similar to that in the higher forms. Amphimixis has been shown to be widespread even among unicellular organisms, yet not in an _essentially_ simpler mode than among multicellulars. We have seen that even in ciliated Infusorians reducing division obtains, and that of the four nuclei which arise from twofold division of the original nucleus three break up again, and only the fourth, by a further division, separates into a male and a female pronucleus, 'which then complete the amphimixis with the corresponding pro-nuclei of another animal' (compare Fig. 85, 4-7, vol. i. p. 321). This, and the existence of a dividing apparatus and of chromosomes, make the process appear very little less complicated than the fertilization of higher animals. The case is similar even in much lower unicellular organisms, such as _Noctiluca_ (Fig. 83, vol. i. p. 317). In this form and in Rhizopods it is true that reducing divisions have not yet been made out, but their occurrence in the lower Algæ (_Basidiobolus_), and above all in those simple unicellular organisms which give rise to malaria, and their allies, which live as 'Coccidia' in the blood-cells and intestinal cells of animals, leads us to expect that they may prove to be of general occurrence among unicellulars.