THE CHROMOSOMES 175 



and have larger cells than the type from which they 

 may be supposed to have arisen. The most im- 

 portant consideration in this connection is that 

 since through tetraploidy the number of genes is 

 doubled, opportunity is given by further mutation 

 in these genes for an indefinite increase in the 

 number of genes in the course of evolution. This 

 possibility suffices to meet the paradox stated by 

 Bateson, that there may have been no increase in 

 the number of genes in the course of evolution from 

 ''amoeba to man." Bateson suggested as a possi- 

 bility that all that has happened in the course of 

 evolution has resulted from the dropping out of some 

 of the original genes. While "evolution through loss 

 of genes" cannot be refuted as an abstract conten- 

 tion, however improbable, it would still leave un- 

 solved the whole question as to the origin of the 

 genes present "at the beginning." Such a specu- 

 lation rests nominally on an hypothesis concerning 

 the nature of mutation itself (loss of a factor). 



In a few cases tetraploidy has arisen in pedigreed 

 material, and in two cases it has been induced 

 artificially. In the evening primrose, Oenothera, a 

 tetraploid form occasionally arises. It has been 

 called by its discoverer De Vries, O. gigas. The orig- 

 inal type O. Lamarckiana has 14 chromosomes and 

 O. gigas 28. Stomps has calculated tliat it occurs 

 only once in about 1 ()(),()()() times. Several possibl(> 

 ways in which it arises have been sugg(\sted. First, 

 that it arises after fertilization through division of 

 the chromosomes of the zygote unaccompanied by 



