VI. SELECTION AND ISOLATION 385 



quently the effect of the selection is enormous. Thus the wonderful 

 variety of domestic animals and cultivated plants were produced by- 

 artificial selection, whereby the individual that seems most disirable- 

 would be selected out of numerous individuals from generation to 

 generation. 



2. The Effect of Isolation and the Origin of Species 



As repeatedly described, newly acquired patterns of protein struc- 

 ture are so unstable and liable as to return readily to the original 

 patterns. This may hold true also for a selected character. A newh^ 

 appearing character will be determined by a gene having a structural 

 pattern newly acquired, and therefore, when an individual having an 

 extreme character which has been advanced by selection is crossed 

 with an unselected individual, the extreme character will soon be 

 lost. Thus the new pattern of a gene of a selected individual is much 

 weaker than that of the original. 



This fact was clearly shown in Castle's experim.ent above cited .. 

 He found that the extreme color-pattern obtained by selection was 

 lost when the selected individuals were crossed with the normal- 

 Entirely similar phenomenon was likewise observed in Tower's experi- 

 ment with the beetles (68). 



Notwithstanding the fact that mutation is given rise to by irre- 

 versible change of a gene, it is generally accepted that the overwhel- 

 ming majority of characters resulting from mutations are recessive, 

 indicating that newly acquired characters are weak even when they 

 are irreversible, and so it should be expected naturally that characters 

 newly acquired by the reversible, gradual alteration of the genes are 

 always weaker than the original. This must be the reason why iso- 

 lation is regarded as the necessary prerequisite and the inevitable 

 cause of specialization. 



Isolation is considered thus to be indispensable for the development 

 of new characters, but when a new pattern can deviate from the- 

 original so far that the crossing between them becomes impossible, 

 isolation is no more necessary and the new pattern is then a ne\w 

 species. There are thus commonly distinct discriminations between 

 species, no intermediate types existing. 



As accepted generally, ecological factors may play the most im- 

 portant role in the isolation, and when the deviation goes far enough 

 the reduction in fertility will result, thus new species being establi- 

 shed. The reduction in fertility involves interspecific sterility and 

 hybrid sterility. Interspecific sterility may be based upon the produc- 



