ORGANIC EVOLUTION — PHYSICAL 35 



among a species of crustaceaus 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 rccisoii d'etre 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 Weis- 

 ujann remarks, it is difficult to understand how one 

 aged and debilitated individual can be rejuvenated and 

 revitalized by union with another individual eijually 

 aged and debilitated. Another raison d'etre must, 1 

 think, be sought for sexual conjugation. Weismann's 

 hypothesis appears to be the most probable ; he thinks 

 that sexual uuion is very prevalent because by it is 

 produced an increased amount of variability in the 

 offspring. 



On consideration it is ajopareut that of two individuals 

 the one that produces ofispring 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 wdiich produces off- 

 spring 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. 



