40 GENETIC VARIATIONS 



this type. Further discussion of these will be reserved till we 

 take up explicitly the subject of mutations. 



But to conclude the matter with which we are now deal- 

 ing, it is found that a single species may thus consist of a great 

 jiumber of diverse stocks, differing in the way their genes are 

 arranged in the chromosomes, but hardly at all in other ways. 

 It turns out on the whole that the way the genes are arranged 

 in the chromosomes is less stable than had been supposed. The 

 genetic system not infrequently undergoes changes in its 

 structure, in this way giving rise to many slightly differing 

 races. 



To recapitulate then, it is clear that much genetic variety in 

 organisms is due to changes in the groupings and numbers 

 of the chromosomes and genes. Such changes occur in plants 

 and animals that are under study in gardens and laboratories. 

 They cause in many cases marked changes in the characteris- 

 tics of the organisms; in their forms, structures, physiology, 

 and methods of reproduction and inheritance. And different 

 types found in nature or in domestication show differences in 

 their chromosome groups that are exactly such as would be 

 produced in these ways. We here catch nature at work differ- 

 entiating genetic types, producing strains and varieties that 

 differ in their inherent constitutions. We thus know certainly 

 what underlies great numbers of genetic differences among 

 organisms. We are able here to see evolution occurring; at 

 least evolution in the sense of diversification. 



But it is clear that in all these alterations in the combination 

 and grouping of the genes, there is nothing that tends directly 

 to bring about adaptation. In the formation of new combina- 

 tions that accompanies biparental reproduction, and in the 

 new grouping of chromosomes and genes that results from 

 irregularities in the operation of the genetic system, unadap- 

 tive conditions are as readily produced as adaptive ones; 



