248 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



ment must be the result of the fact, that they are effectively 

 isolated one from the other, so that they do not share in the 

 heightening or changing of each others potential variability 

 and each others genotype. 



Two groups of animals separated by an effective barrier 

 may be closely alike pheno- typically, and yet, their genotype 

 may be different. This can be seen whenever individuals of two 

 such groups are crossed. We found the field-rats of Sumatra 

 and Java, which are apparently identical to be different geno- 

 typically by crossing them. 



