among a species of crustaceans which he has observed for a number of years. Sexual union therefore, when it does occur, appears to be an essential condition, not the cause of reproduction, much as a sufficient supply of nutriment or of heat is an essential condition, not a cause. Various hypotheses have been put forth as to the raison d'être of it. By some it is supposed that its object is the maintenance of the specific average—an absurd hypothesis, for the law governing organic beings is that of the "survival of the fittest," not that of the maintenance of the specific average. Another hypothesis is that sexual union causes a rejuvenescence and revitalization, so that by virtue of sexual union Maupas's infusorians when they conjugated grew young and vigorous and were able to continue the race, but when they were unable to conjugate grew old and feeble and perished, and with them the race. But as Weismann remarks, it is difficult to understand how one aged and debilitated individual can be rejuvenated and revitalized by union with another individual equally aged and debilitated. Another raison d'être must, I think, be sought for sexual conjugation. Weismann's hypothesis appears to be the most probable; he thinks that sexual union is very prevalent because by it is produced an increased amount of variability in the offspring.
On consideration it is apparent that of two individuals the one that produces offspring that vary within certain limits more from itself is the better placed as regards the ultimate survival of its descendants, other things equal, than the other individual which produces offspring that vary less, assuming of course, what must on the average happen, namely, that some of the offspring vary favourably as compared to the parent and some unfavourably. Though some of the descendants of the first, because they have varied extremely unfavourably,
