Chapter 12 



Evolution by Jumps 



NEW plants and animals with qualities especially 

 selected to meet our purposes have become com- 

 monplace. We have different grains for different 

 soils and different climates, as well as for different propor- 

 tions of starch and protein. We have sheep for particular 

 qualities of wool and sheep for the most economical produc- 

 tion of mutton. We have cows for quantity production and 

 cows for high butter yield; and studies now in progress 

 promise for the near future a new type of cow that will 

 combine the high yield of the Hereford type with the high 

 butter fat of the Jersey type. These practical results are the 

 living proofs of the biological theories regarding the process 

 of evolution, of constant modification of the world's in- 

 habitants. They are the living evidences of the effective- 

 ness of certain kinds of " selection." In every case the 

 breeder had to start with something given — a departure 

 from the conventional structures and functional mode of 

 the race, a variation. But variations are always present. 

 Are they always available for the establishment of new 

 strains or races — new species? 



We have seen that the facts force us to make a distinc- 

 tion between fluctuating variations which result from slight 

 variations in the conditions of growth and development — 

 the phenotypes of Johannsen — and those other variations 

 which seem to arise from more deep-seated sources in the 

 germinal substance of the race — genotypes. 



