No. 567] EFFECT OF DISTRIBUTION ON SPECIAT10N 157 



of lower systematic groups out of proportion to the in- 

 crease of higher systematic groups as the area considered 

 is enlarged is a remarkably constant and wide-spread 

 phenomenon. 



4. The theoretical explanation here proposed for this 

 phenomenon involves a number of complex problems 

 relating to evolution and speciation, including isolation, 

 the time element, and causes of specific and generic 

 modification. 



5. Isolation is an important factor in speciation, since 

 the separation of species into two or more parts allows 

 the parts to become different. The degree of divergence 

 of the segregated parts is largely dependent upon the 

 duration of segregation. 



6. Time, in conjunction with isolation and evolution, 

 tends to increase the number of genera and species in a 

 family, but the index of modification, i. e., the average 

 number of species per genus, remains approximately the 

 same in a given area. 



7. Three types of modifications in animals may be 

 named:— first, ' 1 extrinsic" modifications, which are in- 

 duced by climate and other environmental conditions, and 

 which lead to differentiation of species and subspecies 

 primarily; second, ' 1 intrinsic" modifications, which are 

 concomitant with a change in habits or mode of life of the 

 animal, due to the occupation of a new ecologic niche, and 

 which usually lead to generic or family differentiation; 

 and third, neutral modifications, which are merely the 

 result of the natural tendency of all animals to vary and 

 to be subject to more or less orthogenetic evolution,— 

 modifications which can not be correlated with environ- 

 mental conditions, nor with a change in mode of life of 

 the animal, but which may be influenced largely by in- 

 herited tendencies. Such modifications are responsible for 

 the production, through isolation, of different species to 

 live under the same climatic and environmental conditions, 

 and of different genera to occupy the same ecologic niche. 



8. Specific modifications may be of three kinds : (1) ex- 



