520 
G. H. GKOSVEXOR AND GEOFFREY SMITH. 
which normal!}’ produce only one or two epidemics of sexual 
forms in the year may be beyond the power of the experi- 
menter, and maybe dependent on an internal rhythm such as 
Weisman sn"o-ests. Probably for each species of Cladocera 
this internal rhythm exists, by which the parthenogenetic or 
sexual nature of the successive generations is determined, 
but it is now certain that this determination ” is of a plastic 
nature, and subject to radical modification in response to 
changing conditions. The degree of plasticity, and the 
extent to which the normal life- cycle may be modified, pro- 
bably differs very greatly in different species. Scharfenberg 
(3), who has recently worked with Daphuia magna, admits 
the influence of external conditions, but holds that the life- 
cycle is fairly rigidly determined in Weismann’s sense. Issa- 
kowitsch, on the contrary, holds that nutritive conditions have 
a preponderating influence on the life-cycle, and the earlier 
experiments of Kerheve point in the same direction. 
A critical discussion of the literature (see Scharfenberg [3] ), 
is, however, better postponed until further experiments have 
been performed, and especially the nature of the food which 
the various kinds of Cladocera are dependent on has been 
thoroughly investigated. 
Summary. 
(1) According to AVeismann, in the life-cycle of Aloina 
sexual forms should be produced in every parthenogenetic 
generation independently of external conditions. 
(2) By isolating the parthenogenetic females at birth until 
the production of the brood at a temperature of 25° to 30° C. 
the production of sexual forms is entirely suppressed, 1167 
parthenogenetic females and no sexual forms having been 
obtained by this means. 
(3) Parallel cultures with related females crowded together 
in the culture-glasses at a temperature of 25° to 30° C. gave 
30 3 per cent, males, and at about 14° C. gave 52’3 per cent, 
males. Isolated females at 14° C. gave 19'1 per cent, males. 
