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THE AMERICAN NATURALIST [Vol. XL VIII 



Another law of heredity is that as long as free crossing is main- 

 tained between the different forms of a species these forms can 

 not become widely divergent. The elephant and the mouse could 

 never have been developed from one original stock while free 

 crossing continued. 



Now there are many ways by which the free crossing of one 

 variation with others of the same species may be prevented, but 

 they all come under two groups. 



Under selection are classed all the influences enabling certain 

 variations to reproduce more successfully than other variations, 

 and so preventing free crossing between the successful and the 

 unsucessful. Under isolation are classed all the influences that 

 prevent living, and sexually reproducing creatures, from freely 

 crossing. 



Under normal conditions there is no crossing between the ass 

 and the horse, though there is reason to believe that the early 

 ancestors of each were of one stock freely interbreeding and pro- 

 ducing fertile offspring. If isolation had not existed for ages be- 

 tween them, they could not have become the separate creatures 

 that they now are. Heredity can combine only compatible char- 

 acters. In some cases, incompatible characters arise between 

 creatures of the same race preventing any crossing between them, 

 as when a dextrally twisted mollusk produces a sinistrally twisted 

 one; but, in most cases, such incompatibility arises only after 

 isolation, through geographical separation, for many generations. 



In view of these facts, is it not plain, that, in the case of a 

 variable and plastic organism, races more or less divergent will 

 be produced, if for many generations the organism is divided 

 into branches that are prevented from crossing? Is not such a 

 result just as sure as the gradual transformation of the race 

 under a slow change of climate, when the successful variations 

 are prevented from crossing with the unsuccessful variations? 



John T. Gulick 



Honolulu, T. H. 



