NO. I DISTRIBUTION OF THE ONYCHOPHORA — CLARK 9 



rapidly from those capable of supporting the type to impassible 

 physico-economic barriers. 



It is evident that the individuals at the periphery of the area of dis- 

 tribution, living within a very narrow physico-economic radius, would 

 have to restrict themselves within a very small structural compass, 

 while those at the center of the area of distribution, existing in a very 

 wide economic and physical radius, could wander very far away from 

 the optimum structural condition without meeting prohibitive ob- 

 stacles. 



At the periphery of the range the physico-economic belt capable of 

 supporting the type is so narrow that it serves only as the habitat of 

 a single type, a type which will therefore maintain itself near the 

 original type of the organism. Here additional types cannot arise 

 in any one locality, though slightly different types will be .found in 

 adjacent localities each one of which differs slightly in its physical and 

 economic characters from the others, but all of which are included 

 within the narrow mean. 



At the center of the area of distribution the physico-economic belt 

 is very broad, and it grades imperceptibly away from the mean in 

 either direction. Thus here the original type, instead of being pre- 

 served intact as at the periphery, will eventually be supplanted by a 

 type of subsequent origin, and this latter type will be the one which of 

 all the derivative types is capable of covering the maximum number of 

 economic units. 



The appearance of such a type, which is represented by the dom- 

 inant type seen in each genus, family, and higher group, is inevitable ; 

 for the original type, occupying the mean of the conditions at the 

 center of distribution, will gradually colonize all possible conditions 

 departing from the mean in every direction, this being rendered easy 

 by the very gradual changes from the optimum, and the very wide 

 separation of the impassible physico-economic barriers. The colonists 

 will be more or less modified to suit their new surroundings and, if 

 the physico-economic belt be broad enough, will divide themselves 

 into new types and subtypes. Eventually a type is certain to appear 

 which will alone be capable of occupying all of the regions occupied 

 by the organism as a whole, and which therefore will gradually sup- 

 plant and finally exterminate all the other types ; and this type will 

 not be a primitive type, such as that which is maintained intact at the 

 periphery of the area of distribution, but a much more specialized 

 type; for though the mean of the conditions which it covers is the 

 same as the physico-economic range in which the peripheral types 



