3IO ANIMAL AGGREGATIONS 



vidual to become a female and for those appearing later in the im- 

 mediate vicinity to become males has survival value. Baltzer con- 

 sidered this point in 19 14. He reports that Bonellia is not an abun- 

 dant animal, and occurs in numbers in only a few places in the Bay of 

 Naples. If one supposes that the males remained parasitic but, un- 

 like their present condition, were unalterably predetermined for 

 maleness, then a male larva would be useful only if it found an 

 adult female. All males which did not by chance come into the 

 neighborhood of an adult female must die without issue. This would 

 include a large number of larvae, not only because the species is 

 relatively rare, but because the larvae are positively phototactic at 

 the beginning of their free period and frequently swarm away from 

 their egg mass, even though they are able at this early age to 

 attach to the proboscis of a nearby female. (It appears that these 

 larvae may not attach to the proboscis of their own mother.) By 

 means of their special method of sex determination this potential 

 loss of large numbers of males is avoided, since if they reach a suit- 

 able environment lacking females they develop directly into females 

 and are able then to effect the transformation in the male direction 

 of the next larvae to arrive in that vicinity. The survival values of 

 the Crepidida sex situation are similar. 



Essentially the same situation is found in Cladocera. With these 

 animals the tendency to change, as the culture becomes crowded, 

 from parthenogenetic eggs rapidly produced but not resistant, to 

 the sexual eggs which can withstand adverse conditions, has sur- 

 vival value. In nature such crowding usually precedes the drying-up 

 or freezing of small bodies of water in which the animals have been 

 living; and either usually follows a long reproduction period which 

 has given time for the increase of the cladoceran population to ef- 

 fect a definite change in the environment. Under these conditions 

 the production of resistant eggs has definite value. 



It appears, therefore, that in 4 out of these 5 cases, in which 

 crowding is known to affect the sex ratio, the result has survival 

 value. 



