for the entrance of the male cell. This proves, it seems to me, that this particular spot of the capsule is hereditarily determined, just as much and just as definitely as the ray of the flint-skeleton of a Radiolarian. But if any spot of the capsule can vary by itself alone, may not numerous other points in the animal also be hereditarily determinable? With such complexity of the invisible structure it would not greatly surprise us if we should find amphimixis occurring in all unicellular organisms, and in many of them at a high level of elaboration. These apparently lowly and simple organisms are obviously very far from being the lowliest and simplest, as we shall discover later in a different connexion. But that amphimixis is found as a periodically recurring process even among these, must depend upon the fact that here too the preservation of the best-adapted structure, as well as adaptability to new conditions, requires that the best variants of many different parts of the cell should be brought together, and since the hereditary substance lies in the ids of the nucleus, the union of the ids of two unicellulars will make harmonious and many-sided adaptation materially easier. It will thus give an advantage in the struggle for existence, and we may therefore expect to find that the nuclear substance in all unicellular organism is made up of ids. [22] Zehnder, _Die Entstehung des Lebens_, Freiburg-i.-Br., 1899. [Illustration: FIG. 123. Conjugation of _Coccidium proprium_, a cellular parasite of the newt (_Triton_), after Siedlecki. _A_, a microgamete (_Mi_) in the act of penetrating the shell of a macrogamete (_Ma_) through the micropyle. _B_, the male and the female nuclear constituents are uniting (♂ _chr_ and ♀ _chr_).] The observations hitherto made do not, however, appear to bear this out, for in the lower Flagellata and Algæ the nuclear substance does indeed consist of chromatin, but--as far as it can be made out--of a compact unarranged mass of it. But even though deeper investigations should succeed in demonstrating chromosomes in many of these, the nucleus _must have arisen at some time_, and we must assume that it did so through a more intimate union of previously loose aggregates of determinants, which were gradually arranged and bound together by the combining forces (affinities) we have assumed to obtain among them, thus giving rise to the first chromosomes or ids which were complete in themselves. Then came the multiplication of these ids by the process of division, and only then was the state arrived at from which amphimixis, as we now know it, could have arisen, namely, the existence of a considerable number of identical ids, half of which could be exchanged for the identical ids of another individual in conjugation.