A PHYSIOLOGICAL CHARACTER. 7 



made in October 1912, in Pond I, and in Pond IV in November 1913. 

 The Simocephalus exspinosus material used in selection experiments 

 was obtained from Pond IV, August 1912 and December 1914, and 

 from Pond I, October 1912. 



The stock obtained from Pond I, the surface-water pond, had 

 in all probability only recently undergone sexual reproduction. The 

 pond ordinarily remains dry from early summer until October or 

 November. If as much as 3 to 5 weeks had elapsed between the 

 filling of the pond and November 16, when the latest collection from 

 this pond was made, the daphnids could have descended at most 

 only one or two generations, allowing several days for the (fertilized) 

 ephippial eggs (produced before the pond became dried up in the 

 spring) to develop and 2 weeks for each generation at out-door 

 temperature at that season. It is indeed quite probable that the 

 individuals collected were themselves ex-ephippial individuals. 

 There is no safe criterion for determining how long the material 

 obtained from Pond II (the spring-fed pond) may have reproduced 

 parthenogenetically since the stock had last undergone sexual repro- 

 duction previous to its being taken into the laboratory. However 

 in this pond and in other small ponds in which this species has 

 been observed it has not ordinarily been found to occur for more 

 than 3 to 5 weeks at a time, so that probably this material had 

 comparatively recently descended from ephippial eggs. There is 

 likewise no way of determining how recently the stock collected from 

 Pond IV had undergone sexual reproduction. D. longispina occurs 

 in this pond occasionally, and, so far as observations go, seems not 

 to continue there long at a time. S. exspinosus, however, is found 

 there the year round. No males or sexual eggs of either species have 

 been found in this pond. 1 



The five lines of S. exspinosus originated from five different 

 mothers collected from two ponds and at three different times. 

 There is some evidence (see page 123) that Line 757 (from Pond I, 

 October 1912) at the beginning of selection differed in its reactive- 

 ness from Line 740 (from Pond IV, August 1914). The progenitors 

 of these two lines were obtained from different ponds and with a 

 time-interval of about two months. The other lines (794, 795, and 

 796, from Pond IV, December 1914) of this species seemed not to 

 differ in their reactiveness from each other or from Line 740 obtained 

 two years earlier from the same pond. Possibly these four lines 

 belonged to the same clone, while Line 757 belonged to a different 

 clone. It is possible that sexual reproduction may not occur in the 

 pond from which these four lines (740, 794, 795, and 796) were 

 obtained, and they may all have come from a commoD progenitor. 

 While the material was not examined with this point in mind, 



1 For the purpose of these experiments it is presumably quite immaterial at what time the 

 Inst previous sexual reproduction had occurred. 



