198 INBBEEDING AND OUTBBEEDING 



sexual reproduction through millions of years ; there are 

 asexually reproduced species of plants just as constant 

 and probably still more ancient. At the same time, ger- 

 minal variations occur to-day under sexual reproduction 

 in somewhat noteworthy numbers, as Morgan's work on 

 Drosophila shows. There has been no trustworthy esti- 

 mation of their frequency within even a single species, but 

 it cannot be said they occur in less numbers than where 

 asexual reproduction rules, even among organisms of a 

 relatively high specialization. If there are those who 

 doubt this statement, let them refer to the huge list of bud- 

 variations in the higher plants compiled by Cramer. 33 

 He will be able to identify there, type by type, class 

 by class, practically all of the variations he is able to 

 discover in the same species in the literature on sem- 

 inal reproduction. 



It is rather odd that this should be the case, for what is 

 being discussed here is not really the frequency with 

 which variations occur, but rather the frequency with 

 which they are detected. And theoretically, the ease of 

 detecting variations ought not to be the same under the 

 different modes of reproduction. If it be granted that 

 changes in the constitution of the chromosomes are direct 

 causes of variations, and that such changes in constitu- 

 tion are equally probable in all chromosomes, it follows 

 that p arth en o genetic individuals having the haploid num- 

 ber of chromosomes should show a larger proportion of 

 germinal variations than members of the same species 

 having the diploid number of chromosomes, because 

 variations of all kinds would be recognizable in the 

 former case, while in the latter recessive variations could 

 not be detected until the first or second filial generation 



