86 GENETIC VARIATIONS 



group of genetic changes is so well worked out as that of 

 "gene mutations," but seemingly the more fully this is 

 worked out, the less convincing becomes its role in evolution. 



The efforts to find evidence of other types of genetic change 

 fall into two classes. In one the evidence is indirect; through 

 examination of the results of evolutionary change, conclusions 

 are drawn as to its nature. In the other, genetic variations are 

 directly observed as they occur in living organisms. 



The indirect methods of obtaining evidence on the nature 

 of evolutionary change may be classified in two main groups, 

 with a third minor group. 



I. In the first main group are attempts to trace the evolu- 

 tionary changes that have occurred in the series of organisms 

 that have lived in the past. 



II. In the second group are studies of the diversities among 

 organisms now existing, with attempts to judge of the nature 

 of the changes that must have occurred in producing these 

 diversities. 



III. The minor group concerns changes that have appar- 

 ently occurred in the genes of natural polyploids. 



With these indirect methods of obtaining evidence on evo- 

 lutionary change it is not the province of these pages to deal. 

 Yet for comparison with the results of direct observation of 

 genetic variations, certain typical lines of evidence may be 

 sketched, with some of the conclusions that have been drawn 

 from them. 



I. To trace the evolutionary changes that have occurred in 

 the series of organisms, from lower to higher, that have lived 

 in successive geological ages, study must of course be limited 

 to the hard parts of organisms, since only these are preserved. 

 The literature of such study is of course enormous. As an 

 example there may be mentioned the work of H. F. Osborn, 



