270 University of California Publications in Zoology [Vol. 18 



remained on the surface. This would, however, require a lower 

 viscocity in cold than in warm surface water, for otherwise aggregate 

 forms would be most abundant in warm surface water and least 

 abundant in cold surface water, which is contrary to fact. But even 

 so, it would be physically impossible for solitary forms or short chains 

 to remain in maximum numbers on the surface when aggregate forms 

 or long chains were present in minimum numbers. For, the former, 

 being heavier than the latter, must sink if the latter do. 



The third assumption (c) is, perhaps, the most probable, because 

 since many individuals of the aggregate generation contain an embryo 

 of the solitary generation, it may well be that their specific gravity 

 exceeds that of the solitary forms and, for the same reason, the specific 

 gravity of long chains may actually exceed that of short chains. Under 

 these conditions a decrease in viscocity induced by an increase in 

 temperature might permit the sinking of aggregate forms or long 

 chains while solitary forms or short chains remained on the surface. 

 This might explain why aggregate forms were most abundant and most 

 frequent in the colder surface water and least abundant and least 

 frequent in the warmer surface water. But, as in case (b), solitary 

 forms could not be more abundant in warm than in cold surface water, 

 for cold water, being denser, anything heavy enough to sink in cold 

 water must certainly sink in warm water. 



Again if it be claimed that, owing to variation in evaporation, 

 warm surface water is at times denser than cold, while at other times 

 cold surface water is denser than warm, what must the consequences 

 be? Returning to the three alternative assumptions, if (a) were 

 true, the warmer water must have been the more dense during the 

 majority of hauls or solitary forms could not have been taken in 

 greater numbers from the warmer water. Yet, to obtain the aggre- 

 gate forms in greater numbers from the colder surface water, it 

 would have had to be the more dense' during the majority of hauls. 

 Obviously both conditions could not have been realized, and even if 

 they could, the frequency of solitary forms would have paralleled their 

 abundance, which is contrary to fact. 



If either (b) or (c) were true, the situation, while more complex, 

 remains essentially unaltered. For in the first case, solitary forms 

 or short chains, being the heavier, would of necessity sink whenever 

 aggregate forms or long chains sank; while in the latter, aggregate 

 forms or long chains, being the heavier, would have to sink whenever 

 solitary forms or short chains sank. 



