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Mr. G. Smith. 



take some instances in Generation 6, 65 crowded parents gave 415 partheno- 

 genetic young and no males ; in Generation 37, 20 crowded parents gave 222 

 parthenogenetic young and no sexual forms. It must be concluded from this 

 that there is some factor involved in the production of the sexual forms other 

 than external conditions, viz., an internal factor. That this internal factor, 

 is a regular rhythmical cycle, such as Weismann originally suggested (1), which 

 runs on without any regard to external conditions, is obviously not true, but 

 there is this very important element of truth in Weismann's view, namely, 

 that a species such as D. pulex never produces as many sexual forms per 

 cent, as a species like M. rectirostris ; and, as far as we know, no alteration of 

 the external conditions would make it do so. The facts suggest that for each 

 species of Cladoceran there is a maximal limit to the numbers in which sexual 

 forms may be produced, and that this number cannot be readily increased : but 

 it can be decreased, or entirely abolished, by external conditions such as isola- 

 tion and high temperature combined with abundant nutrition. If we suppose 

 with Woltereck (3) that the production of parthenogenetic and sexual forms 

 is due to the presence of two substances, a parthenogenetic substance and a 

 sexual substance, then we should say that the relative amounts of these 

 substances are fairly rigidly fixed for each species, and that, whereas the 

 amount of the sexual substance cannot be easily increased, its operation can 

 be indefinitely suspended by the action of external conditions. By this inter- 

 pretation of the facts it is possible to retain the really important part of 

 Weismann's theory, that the proportions in which the sexual forms are 

 produced in each species is fixed in its upper limit in accordance with the 

 adaptive necessities of the species, though we must maintain that these 

 proportions can be altered by the suppression of the sexual forms through 

 external conditions. 



W. E. Agar (7) in a recent paper, after summarising the results of previous 

 workers, comes to the conclusion that there is no such thing as an internal 

 rhythm in Cladocera, and that the production of sexual or non-sexual forms 

 is entirely controlled by the environment. I am in agreement with Agar in 

 thinking that there is no hereditarily fixed rhythm, or that the production of 

 the sexual forms is rigidly fixed on to particular generations or particular 

 broods of these generations, but I find it impossible to believe that it is purely 

 due to environment that a species like M. rectirostris, under any conditions, 

 produces a far greater percentage of sexual forms than a species like D. pulex. 

 In other words, there is an internal factor concerned in the production of 

 the sexual forms ; this factor varies in different species of Cladocera ; its 

 operation can be entirely suspended by external conditions so that no sexual 

 forms are produced ; but there is no experimental justification for the view 



