188 Reproduction in Simocephalus vetulus 



Mr John Ritchie. From the Beith specimen 17 generations were taken 

 and from the Cambridge specimen 16 generations, giving with the 

 original females 18 in the Beith and 17 in the Cambridge lines. They 

 were kept under exactly the same conditions as the main line, and in 

 neither of them were any sexual forms found at all, nor any appearance 

 of "degeneracy," though as described above a very large proportion of 

 the eggs of the first broods of the contemporary thirteenth generation 

 of the Cambridge, and fourteenth of the Beith lines failed to develop. 



It again happens that if the period during which these lines were 

 bred is divided into two portions, the mean size of the young in the 

 earlier generations is less than in the later ones. Combining the con- 

 temporary generations of the two lines together we find that the mean 

 length of the first brood young of the first period (seven generations 

 measured) is 45*256, and of the second period (also seven generations 

 measured) is 45*904. A closer analysis shows however that it would be 

 again quite unjustifiable to conclude that a progressive increase was 

 taking place. If we arrange the generations in order of the magnitude 

 of the new-bom young, we find the order, beginning with the smallest, 

 is (generations 1 and 9 not being available) 



8, 5, 16, 2, 11, 17, 4, 3, 10, 14, 6, 12, 15, 7. 



In a fairly extensive experience of breeding Cladocera I have of 

 course often experienced the phenomena of bursts of sexuality and 

 of " degeneracy " — or better, of high mortality. I have had both these 

 phenomena appearing in D. pulex, and the sexuality in iS. exspinosus 

 kept under the conditions described as normal above, and also in 

 8. vetulus kept under other conditions. 



It is obvious therefore fi'om this and fi-om the work of others that 

 the same species may under certain conditions exhibit an increasing 

 tendency to sexuality and degeneration ending in total cessation of 

 parthenogenetic reproduction, and under others may continue the 

 asexual reproduction apparently indefinitely. My main line was to all 

 appearance as vigorous at the 46th generation as at the first, and so 

 were the Cambridge and Beith lines as long as they were bred — and it 

 must be remembered that the parthenogenetic females which originated 

 the three lines may themselves have had a long parthenogenetic 

 ancestry. There is no justification for the supposition that if the 

 experiment had been continued "long enough" the lines would have 

 at last been unable to maintain themselves without sexual repro- 

 duction. 



