484 ZOOLOGY 



of dogs, pigeons, chickens, and many other domestic animals 

 have apparently arisen. None of these things could happen but 

 for the fact that animals can vary. These results are not merely 

 a proof of evolution ; they are evolution. Any one who believes 

 in these changes is an evolutionist by just that much. 



There can be no reasonable doubt that just this kind of 

 thing is happening in nature, without the help of man. It 

 cannot take place, however, so rapidly as when man deliberately 

 aids the process, by artificial selecting and breeding according 

 to his preference, and then eliminates those that he does not want. 

 It sometimes happens, both in nature and in cultivation, that 

 large variations ("sports") appear suddenly, and breed true in 

 succeeding generations. These marked variations are not so 

 frequent as the slighter ones, but seem to be more persistent 

 when they once appear. Whether large or small these heritable 

 changes in animals, arising from changes in the germ plasm are 

 known as mutations. 



482 Evidences from Geographical Distribution. — In the 



wild state, animals usually change too slowly for us to detect 

 that there has been any change in a species. Accurate record of 

 measurements extending through several human generations, 

 would be necessary in order to demonstrate permanent changes ; 

 yet we do have some evidence on a broader scale which is of a 

 nature quite similar to that in the last section. In the way in 

 which animals occur on the face of the earth a great many inter- 

 esting factors enter, and it sometimes happens that we get some 

 good indication of the long-time effects of variation. Sometimes 

 we find two forms of plant or animal flourishing in two regions 

 that are separated from each other by some kind of a barrier 

 which does not allow them to pass back and forth and thus to 

 mingle. These forms are in general similar, and yet are con- 

 stantly and recognizedly different. Sometimes we can find 

 that these two regions (b and c)' have been stocked from some 

 third region (a) , and that both varieties are apparently descend- 

 ents of the same ancestors. Indeed, we may be able to find 

 specimens of all grades connecting "a" with "b" on one side, 

 and other intermediate types connecting "a" with "c" on the 



