CROWDING AND SEX DETERMINATION 309 



resulted. Much evidence has accumulated of recent years that sex 

 determination at or before fertilization is not the hard and fast fact 

 it was regarded to be in the first blush of enthusiasm over the dis- 

 covery of the sex chromosomes. The work of Lillie, Goldschmidt, 

 Crew, and Domm has shown a much greater fluidity than was at 

 first anticipated in the light of the chromosome theory. The results 

 of metabolic effects produced by crowding, reported here for mon- 

 striUid copepods and Cladocera among the Crustacea; for Bonellia, 

 the aberrant annelid worm; Crepidula, the gasteropod mollusk, and 

 certain of the nematode worms, are not out of harmony with other 

 instances of epigametic sex determination produced by other means. 

 From our present knowledge it appears that sex determination, at 

 least for the majority of animals, is normally associated with the 

 chromosome mechanism, but that chromosome determinations may 

 be overruled by other factors, among them the effects produced by 

 crowding. In certain cases these effects appear to be produced by 

 the transfer of material from one individual to another, as in 

 Bonellia; in others, as Cladocera, by the effects of crowding upon 

 animals that are not necessarily in physical contact with each other. 

 From our point of view the important thing is that they occur at 

 all and can be controlled, in certain animals of widely distributed 

 taxonomic position, by the degree or kind of aggregation obtaining. 



When we examine the possible survival value of the effect of 

 crowding upon sex, we find a mixed situation. In the monstrilHd 

 copepods crowding has negative survival value, in that a normal sex 

 ratio among uncrowded animals gives way to an overproduction of 

 males among crowded individuals. With the parasitic nematodes 

 the high proportion of males accompanying heavy infestations would 

 appear to be as definitely harmful for the race as their absence in 

 cases of low infestation. Here is evidence of an optimum popula- 

 tion, where males and females are in fairly equal ratio, well above 

 the minimum population, as we have seen in other relationships. 

 It may be suggested that there is a more remote survival value 

 in the overproduction of males, in that the neighborhood is less 

 likely to be overstocked, to the destruction of all. 



With Bonellia and Crepidula the tendency for an isolated indi- 



