DEPARTMENT OF EXPERIMENTAL EVOLUTION. 121 



for 145 generations, and 3 for 22 generations produced males in all but 

 one line. With Simocephalus vetulus 12 of the 16 strains, some of which 

 had been in the laboratory for 188 generations, the others for 81 to 97 

 generations, produced males; 3 lines of Simocephalus serrulatus reared 

 in the laboratory for 65 generations and 10 lines — one from Lakeland, 

 Florida, and 9 from Eustis, Florida — reared for only 4 to 6 generations 

 responded almost simultaneously in the production of males. With 

 Moina the story is the same; all the lines produced males at once. 



Such synchronism in independently reared and diverse stock can be 

 thought of only in terms of a common influence. It seems a logical 

 impossibility that this synchronism could be due to an internal factor, 

 for it seems hard to believe that in 42 out of 52 strains of quite diverse 

 material the appropriate periods for production of males in an innate 

 sexual cycle should occur almost simultaneously. One must then ascribe 

 the synchronism to an external — an environmental — factor or, perhaps, 

 to a number of such factors. 



It was noted in last year's report that in cases in which males had 

 been observed environmental changes had obviously occurred. In the 

 present case the environmental influence was strongly in evidence. In 

 the outdoor pond from which the culture-water was obtained males 

 were occurring in great numbers in the wild stock — just as in the few 

 earlier and very restricted occurrences of males in the laboratory, males 

 had been noted in the wild stock in the outdoor ponds from which the 

 culture- water was gotten, or else (in the absence of wild stock in the out- 

 door ponds) the culture-water was recognized as somewhat different 

 from the material ordinarily used. During none of the earlier occur- 

 rences of males in outdoor ponds had there been such a great number 

 of males either in the ponds or in the laboratory as during the past 

 spring. During the recent epidemic of males the environmental influ- 

 ences tending to produce males were very much more pronounced on 

 the wild stock as well as upon that reared in the laboratory. 



Experime7ital evidence. — Direct experimental evidence adds to the 

 conviction (if additional evidence were necessary) that environmental 

 factors cause the production of males. 



In one of these experiments the newly released young of a single 

 brood from one mother were used. These were isolated in individual 

 bottles — 5 in culture-water from a pond in which were present great 

 numbers of males of another species of Cladocera, and 4 in water from 

 a pond in which no males were present. None of the latter 4 produced 

 any males ; all of the 5 in food material from the pond in which occurred 

 wild males produced males. 



In another experiment, similarly conducted, food from two sources 

 was used. Different bottles of each food were given different chemical 

 treatment. A small amount of alkali was placed in some and a small 

 amount of acid in others. Still others were untreated. The result 

 was as shown in table 5. 



