328 Genetics of a Daphnia Hybrid during Parthenogenesis 



unity for each degree of ancestry). Genetic diversity, although it existed 

 between different clones, did not arise between parent and offspring, as 

 shown by the fact that offspring do not resemble their parents more 

 closely than they do their great-great-grandparents. The characters of 

 the individuals of each generation are determined, apart from temporary 

 environmental modifications, solely by the genotype of their clone, and 

 not in any way by the individual characters of their parents or of any 

 other particular ancestor. 



This conclusion was completely confirmed by a parallel experiment 

 in which the population was composed of 69 lines descended from the 

 same number of females, not however derived from separate fertilized 

 eggs, but all descended parthenogenetioally from a single known ancestor. 

 The constants calculated from this monoclonal population are reproduced 

 in Table IX for comparison with Table VIII. It will be seen that the 

 coefficients are some negative and some positive, but also sensibly zero. 



TABLE IX. 



Mea/n correlation coefficients /or the monoclonal population o/S. exspinosus. 



Size at Birth Size at Maturity 



Parental --025 --007 



Grandparental -014 ~ -062 



Great-grandparental ... ... -'098 - -llS 



Great-great-grandparental ... '066 — 



Thus no inheritable differences existed or arose among the members 

 of the monoclonal population. 



As pointed out in the paper from which these two tables have been 

 extracted, the results imply absence of Mendelian segregation in the 

 parthenogenetic reproduction of the Cladoceran, S. exspinosus. The ex- 

 periments described in the present paper contain direct confirmatory 

 evidence on this point. 



What for convenience may be termed the genotype theory of heredity 

 rests on the fundamental hypotheses that (a) the hereditary material 

 consists of relatively stable discrete units located in the chromosomes, 

 and (b) that genetic diversity between parent and offspring is due to one 

 of two causes : either to mutation, or, much more frequently, to the segre- 

 gation of homologous chromosomes in meiosis and their recombinations 

 at syngamy. The latter part of this dual thesis is capable of direct 

 experimental test. If genetic diversity (except for mutation) is due to 

 the processes of meiosis, it should not arise in any form of reproduction 

 in which meiosis does not occur. This test has been applied, with success 



