But this cannot have been the efficient cause in the introduction of amphimixis into the series of vital phenomena; the reason for this must be found in some _direct_ advantage, such as that it improved and increased the assimilating power, the growth, and the multiplication of the particular individual, so that it gained an advantage over individuals which had not entered into conjugation. This advantage must exist, at least in the lower forms of conjugation, in pure plastogamy, i. e. in the mere coalescence of the protoplasmic bodies. But, as it seems to me, we have not yet clearly recognized what the advantage precisely is; we do not yet see how such a mingling or combination of two plasms should every time be of advantage for the combined conjugate. If we assume with Zehnder that two kinds of 'nutritive' biophors are brought together which differ slightly from each other in digestive capacity, three cases may occur. Either the food _a_, adequate for the animal _A_, is just as abundant as the food _b_, suitable for the animal _B_, and then half the conjugated animal will be nourished by means of the biophors _a_, the other half by means of the biophors _b_, and the state of matters is the same as it was before conjugation; or the food _b_ is more abundant than the food _a_, or conversely, and then the biophors _b_ will have to take the larger share in the nourishment of the conjugate _A_ + _B_, and they will therefore multiply more rapidly and the biophors _a_ will decrease relatively in number. Nutrition and growth will then go on more slowly for a time, but will soon attain to their former intensity. The combined individual _A_ + _B_ has then certainly gained an advantage over the isolated animal _A_, and the living substance of _A_ which, if left to itself would probably have perished, can continue to live in combination with _B_. But in that case it is not obvious where the advantage in the union can lie, as far as _B_ is concerned. An advantage to _B_ only results if there be a combination not of _one_ kind of biophor only, but of several or many kinds of biophors. If for instance _A_, whose digestive biophors were weak, brought with it into the partnership 'secretory' or nervous biophors stronger than those of _B_, then there would be an advantage for both in the combination, and it is thus that, in the meantime, I interpret the direct benefit which results from pure plastogamy. This benefit must be the more important and far-reaching the longer multiplication by fission continues without the occurrence of conjugation.