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THE AMERICAN NATURALIST [Vol. XLVIII 



In the past, much of the work that has been done on 

 zoogeography has dealt with a study of the facts of dis- 

 tribution, both present and past, as they stand, together 

 with a study of the factors influencing distribution and 

 speculations regarding the explanation of some of the 

 interesting and apparently anomalous facts thus brought 

 to light. In all of this work, the distribution of animals 

 has been considered almost entirely as the effect of cer- 

 tain biological and geological causes. The present paper 

 is intended to show that the distribution of animals is not 

 only the effect of other causes, but is in itself the cause 

 of other effects, and that extent of distribution has a 

 direct influence on the modification and speciation of the 

 group concerned. 



To find out how far-reaching and how potent is this 

 effect, much further study is necessary, not only of the 

 distribution of various groups, but of their classification 

 and systematic relationships as well. 



In brief, the effect of extent of distribution on groups 

 of different systematic rank may be stated as follows: 

 As the range of a group of animals, be it genus, family, or 

 order, is extended, the species increase out of proportion 

 to the genera, the genera out of proportion to the families, 

 and the families out of proportion to the orders. In 

 other words, if we assume that in a distributional area 

 of certain extent, there are three genera and six species, 

 in a distributional area of twice that size, there will not 

 be six genera and twelve species, but more probably only 

 four or five genera, and twelve species ; i. e., if in the first 

 case the index of modification (a term here used to indi- 

 cate the average number of species per genus) be two, in 

 the second case it will be greater than two. 



As new distributional areas are added, other factors 

 remaining equal, there is a constant increase in number 

 of species and subspecies, going hand in hand with a 

 diminishing rate of increase in genera, the result being a 

 constantly larger index of modification as the area in- 

 habited by a group of animals is extended. 



