No. 567] EFFECT OF DISTRIBUTION ON SPECIATION 155 



a family of animals, by adaptive evolution, will approach 

 a maximum of generic differentiation which can be sup- 

 ported in that area. In other words, every suitable eco- 

 logic niche which is represented in the region considered 

 will be invaded by the family, and even in a small area 

 there is likely to be a considerable generic differentiation, 

 especially if isolation has had any opportunity to operate 

 within the area, in breaking up the genera and species. 



Let us assume that in one unit of area a certain family, 

 Sciuridae for example, was represented by three genera, 

 each with three species. Second, let us assume that this 

 family kept spreading into additional units of area. With 

 each new unit, the chance of new suitable ecologic niches 

 being represented would decrease, and therefore the 

 chance of new genera being represented would decrease, 

 since if a genus were fitted for its niche in nature under 

 certain conditions of climate and environment, it would 

 in the majority of cases not be likely to undergo any 

 radical changes in the occupation of the same niche 

 under somewhat altered conditions of climate and environ- 

 ment; i. e., the stimulus for intrinsic modification would 

 be lacking. 



On the other hand, with each additional unit of area, the 

 chances of the combined conditions of temperature, hu- 

 midity, and environment being different, would remain the 

 same. In other words, the chances of the three dimen- 

 sions influencing the life of a region, i. e., * * life zone" 

 (controlled by temperature), "fauna" (controlled by hu- 

 midity), and "association" (controlled by the effect of 

 the other two plus a number of other environmental con- 

 ditions), intersecting at the same point would be almost 

 equally improbable with each succeeding unit of area. 

 Since it is changes in "life zone," "fauna," or "associa- 

 tion" which produce extrinsic changes, and therefore lead 

 to differentiation of species and subspecies primarily, the 

 increment of species would average nearly the same for 

 each succeeding unit of area, other factors remaining 

 equal. It should also be taken into consideration that 



