404 HEREDITY AND ENVIRONMENT 



But it is probable that mutations are not 

 usually associated with changes in the number 

 of chromosomes. Where the number of chro- 

 mosomes remains constant the change may 

 take place in the number or composition of 

 the chromomeres or units of the next lower 

 order, but it would be practically impossible to 

 find such changes in bodies so small and so 

 numerous. Whatever the cellular changes may 

 be which accompany mutations, it is certain 

 that changes take place in the inheritance fac- 

 tors. Sometimes factors drop out, as in the 

 white sweet peas shown in Fig. 57, and in many 

 other cultivated races of plants and animals, 

 thus producing regressive mutations. Indeed 

 most of our domestic animals and cultivated 

 plants have arisen by the omission of some- 

 thing which their wild ancestors had. In most 

 of the mutations studied by deVries, Bateson, 

 Morgan and others some factor has dropped 

 out and Bateson suggests that at present all 

 new forms arise only by the loss of factors or 

 by the fractionation of factors and that new 

 factors are not added from without. This leads 

 him to inquire "whether the course of evolution 



