252 Biology in America 



normal paioiits, these whicli wci'c I'crtik' wore more re- 

 sistant (fewer died before hatching) and the chicks hatched 

 were heavier than those of normal chickens. Pearl has ap- 

 parently pnblished no resnlts dealin*? with the effects of alco- 

 holism on {^generations later than the first, but Stockard's ex- 

 periments seem to show that such effects are transmitted to 

 the grandchildren. 



In the light of these contiadictoi'y results no final word can 

 be said regarding the intfuence of alcohol .on animals and 

 the transmission of such influence to subsequent generations. 

 In neither the experiments of Stockard nor of Pearl did the 

 treated animals themselves show any conspicuous effect 

 (apart from blindness in the guinea pigs, which was the di- 

 rect result of the action of the alcohol fumes upon the eye) 

 although the alcoholic chickens increased somewhat in flesh 

 and became rather lazy, a result easily paralleled in some 

 cases in man. 



Recognizing then the occurrence of variations produced 

 by physico-chemical factors either internal or external to the 

 organism itself, and granting that such variations may in 

 some cases be preserved, but probably neither increased nor 

 diminished by selection; are there other factors which may 

 influence evolution ? 



In 1868 ]\Ioritz Wagner suggested the influence of geo- 

 graphic isolation as a factor in the evolution of plants and 

 animals, and this theory has more recently been advocated 

 by David Starr Jordan in this country. 



One of the objections to the theory of natural selection is 

 the swamping effect of intercrossing betw'een nascent spe- 

 cies. How this "swamping" is effected if new varieties are 

 discontinuous or mutating, breed true and do not blend when 

 intercrossed, is difficult to understand. It is possible how- 

 ever that the dominant types being more numerous have 

 caused the reccssives to be overlooked, and that consequently 

 the "swamping effect of crossing" is more apparent than 

 real. If however such objection is valid, the danger to the 

 new species could be removed were a barrier to such inter- 

 crossing to arise between nascent types, thereby preventing 

 them from interbreeding. That such isolation does play an 

 important role in evolution, there is good reason to believe. 



Isolation may be of several kinds: psychical, physiolog- 

 ical, structural, habitudinal and geographical. It is well 

 known that many species of nearly related animals will re- 

 fuse to interbreed. In other cases in which different species 

 of animals do interbreed the offspring are ordinarily infer- 

 tile, perhaps the best known instance being the mule, which 

 is a hybrid of the jackass and the mare. Mulatto women, 



