Abstract
Summary.1. The two series of observations on Cladocera showing (a) that a state of depression produced by unfavourable external conditions in Daphnia females kept in cultures causes a change from parthenogenetic to gamogenetic propagation, and (b) that gamogenesis in nature is accompanied by a state of depression in the populations in question, go to prove the correctness of the hypothesis that in nature the transition from parthenogenesis to gamogenesis is caused by the influence of unfavourable external conditions. Unfavourable external conditions cause states of depression in the females and thereby the change in the mode of reproduction and so also a change in sex determination. (Section II.)2. A number of earlier and more recent laboratory experiments have given results which support this depression hypothesis. (Section III.)3. Investigations in nature on parthenogenesis and gamogenesis in the Cladocera and simultaneous observations on other functions in these animals likewise afford support for the depression hypothesis. (Section IV.)4. The reproductive cycle of the Rotifera is compared with that of the Cladocera. There is a good deal of evidence which would seem to indicate that in the Rotifera transition from parthenogenesis to gamogenesis—and so also the sex determination associated with it—is likewise caused by a state of depression, and that this state of depression may be induced by the influence of various external factors. (Section V.)5. A general survey is taken of some of the biological peculiarities characterising the state of depression associated with the change from parthenogenesis to gamogenesis. (Section VI.)
Published Version
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