VARIATION AND MENDELIAN INHERITANCE 377 



in large numbers, there would, without question, occur such a 

 mingling of types as would suffice to bring about this result. The 

 case would be indistinguishable from that of our combining in 

 the same table the measurements of individuals from different 

 local collections. But it does not seem at all probable that these 

 animals indulge in such extensive migrations within their com- 

 paratively brief lives. It would probably take many generations 

 of mice before the descendants of any particular local strain 

 would reach a point only twenty miles distant, and this would 

 be doubly true if any form of geographic or ecologic barrier 

 intervened. 



Now it is important to point out that such a gradual inter- 

 mixture of two local races, even if complete, would not result 

 in bringing about the correlation of two characters which did not 

 previously tend to vary together.^ For, as will be shown in 

 detail later, there seems to be no tendency in hybridization for 

 these various racial characters to be transmitted together. The 

 'rubidus' mice from Humboldt County have both a consider- 

 ably longer tail and considerably wider tail stripe than the 

 'gambeli' mice from Calistoga. But neither in the Fi nor the 

 F2 generation of hybrids do we find any more evidence of cor- 

 relation between these characters than we find in the pure 

 races, considered separately. The same independence in trans- 

 mission is probably true in respect to tail stripe and foot 

 pigmentation. 



Thus it would seem that the racial complex of characters is 

 permanent only so long as mice of the same 'race' breed toge her, 

 as happens of necessity in nature. There is no linkage among 

 these characters in heredity, or at least this is true of some of 

 the most distinctive ones. 



Such a condition of independent transmission is, of course, the 

 familiar one in Mendelian inheritance, where the various unit 

 factors segregate, for the most part, without relation to one 

 another. But here the resemblance ends. As will shortly be 



^ It would, of course, eliminate all local differences of type, unless the modify- 

 ing agencies continued to operate at a rate sufficient to outweigh the process of 

 diffusion, an assumption which we must make wherever local differences are 

 encountered. 



