290 ANIMAL AGGREGATIONS 



The aggregation phenomenon may be of still greater importance 

 in sex biology than the foregoing illustrations would indicate. The 

 following considerations strongly suggest that it may have been an 

 essential element in the evolution of sex itself. Presumably, this 

 evolution started with a time when all gametes of any one species 

 were similar. Under these conditions a first step toward union of 

 two isogametes could be supplied by the greater well-being fostered 

 by the presence of more than one gamete within a limited area, such 

 as we have seen holds, under certain conditions, with the Protozoa 

 and bacteria. From the survival value thus present before actual 

 union took place, we can find a logical beginning for the action of a 

 selection, which would in time, and with present known values, re- 

 sult in the establishment of the sexual phenomena as they appear 

 today. 



These fields, important as they are, have not yet been explored 

 sufficiently to allow more than this suggestion of their fundamental 

 significance. In the matter of sex determination, however, there is 

 a mass of evidence concerning the importance of the close associa- 

 tion of animals which merits presentation. 



THE EFFECT OF CROWDING ON SEX IN THE 

 MONSTRILLID COPEPODS 



The copepod crustaceans of the aberrant family Monstrillidae are 

 marine and free-living as adults, but in their larval stages certain 

 of them have been shown to be parasitic in the blood-vessels of 

 marine annelid worms. Malaquin (1901) has described in detail the 

 life-history of Haemocera danae, which passes its parasitic stage in a 

 serpuHd worm. The adult males and females are highly dimorphic. 

 Malaquin reports that when a single parasite is found in the host 

 worm it may be either a male or a female, but that when two or more 

 are present they develop into males, with the exception of very rare 

 cases; in two cases only, out of some thousands examined, were two 

 females found in the same host. The crustaceans gain entrance to 

 their host in the nauplius stage, and it is extremely unlikely that the 

 observed sex distribution is the result of a differential penetration of 

 predetermined sexes. Since, when a single parasite is present, either 



