156 THE AMERICAN NATURALIST [Vol. XLVIII 



with the invasion of new zoogeographic areas, contact 

 with allied forms is frequently experienced, and oppor- 

 tunity is thus afforded for cross breeding and hybridiza- 

 tion, the result of which upon the germ plasm appears to 

 be as influential in the production of new forms as is the 

 shock of new environmental conditions. The constant 

 increase in species and subspecies accompanying invasion 

 of new territory, going hand in hand with a diminishing 

 increase in genera, results in the constantly larger index 

 of modification as the area inhabited by a group is 

 extended. 



Summary 



1. Extent of distribution has a direct influence on the 

 speciation of the group concerned in this way, that as the 

 range of a group of animals is extended, the species in- 

 crease out of proportion to the genera, the genera out of 

 proportion to the famines, and the families out of pro- 

 portion to the orders. 



2. Comparison of different families having unequal geo- 

 graphic ranges is usually inaccurate due to the great dif- 

 ferences in the other factors controlling their speciation. 

 Those families which do lend themselves to such a com- 

 parison show decidedly the effect of extent of distribu- 

 tion, e. g., the bats and some of the insectivores, the fami- 

 lies of widest distribution having the largest indices of 

 modification. A number of exceptions exist in the form of 

 certain wide ranging genera which have a paucity of 

 species. We have no adequate explanation for this 

 phenomenon. 



3. Comparison of the faunas of areas of different size 

 gives very accurate results. A number of tabulations show 

 as a whole an invariable increase in the index of modifica- 

 tion as the distributional area is extended by the addition 

 of either life zones, faunas, or associations. Such tabu- 

 lar comparisons were made for all the classes of ter- 

 restrial vertebrates, for several families of insects, and 

 for the marine Amphipoda of the suborder Gammaridea. 

 Allowing for explicable exceptions, the increase in number 



