MENDELISM AND OTHER METHODS OF DESCENT 237 



tion. They do not teach us the principles or methods of normal 

 constructive evolution, except as we learn to interpret them 

 conversely. The discontinuity of variation shown by the origi- 

 nation of these characters of degenerating domesticated varieties 

 finds few complete parallels in nature. Moreover, these muta- 

 tive characters, instead of being nicely preserved in nature, are 

 wiped out by contact with the wild stock of the species, or even 

 by free interbreeding among the domesticated varieties them- 

 selves. The prepotency of recent mutations corresponds to the 

 prepotency of genetic variations in nature, but the evolutionary 

 status of the mutations is very different from that of the genetic 

 variations. 



Discontinuous variations do not prove discontinuous evolution, 

 but makes continuous evolution more practicable and probable. 

 It is true that Darwin and many other believers in selection as 

 the motive power of evolution have dwelt upon the efficacy of 

 accumulating even infinitesimal differences, but there can be 

 no objection to the accumulation of variations of more than 

 infinitesimal size if the facts seem to justify such a view. The 

 fundamental issue is between evolution, or gradual changes in 

 species (whether by smaller or larger steps), and saltation, or 

 sudden jumps from one species into another. De Vries has 

 even been careful to specify that the jumps shall be sideways, 

 on the ground that species once originated by mutations do not 

 change or make further advances, all progress being confined 

 to lateral displacements of mutative variation. 



Mutations and Mendelism are very interesting phenomena, 

 not only for breeders of domestic plants and animals, who have 

 long known them under other names, but in adding, if some- 

 what indirectly, to the evidence that new variations are pre- 

 potent, and that evolution may be accomplished by the spon- 

 taneous accumulation of appreciable genetic variations, instead 

 of depending upon the selective integration of infinitesimal 

 effects of environmental influences. When this distinction 

 between discontinuous variation and discontinuous evolution is 

 once appreciated, it will become apparent that the mutation 

 theory and the Mendelian " laws" which have been enlisted in 

 its support are assumptions which the facts do not warrant. 



