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THE AMEBIC AN NATUBALIST [Vol. XLVIII 



tinental islands, such as Madagascar, Australia and New 

 Zealand. Decreasing degrees of isolation may be observed 

 in our West Indian islands, where some generic differenti- 

 ation has occurred; in the Santa Barbara islands, where 

 there has been a differentiation of species; and the de- 

 tached mountain ranges of Southern California, where the 

 upper life zones are at present in an isolated condition, 

 but have been so only long enough to develop a few new 

 subspecies, and to lose many of the types of the mother 

 range, in accordance with the law proposed by Grinnell 

 and Swarth (1913) that 4 'the smaller the disconnected 

 area of a given zone, or distributional area of any other 

 rank, the fewer the types which are persistent therein. ' ' 



From this it is apparent that the time element, in con- 

 junction with isolation, may have a very decided effect on 

 the number of genera and species in a family, but since, 

 from a geologic point of view, animals appear to have 

 reached a new equilibrium very quickly after a geographic 

 change, the time element may have little effect on the num- 

 bers of genera and species relative to each other in any 

 given area. In other words, as fast as new genera are 

 produced in a given area, the species within the genera 

 will tend to be produced in the same ratio, thus leaving 

 the index of modification unaffected. 



As an example of the effect of time and isolation let us 

 take a hypothetical case. Let us assume that a certain 

 island became divided into two islands of unequal size, 

 and that after a short period of segregation, just long 

 enough for the fauna to readjust itself to the smaller 

 areas and reach a new equilibrium, we had say six species 

 in three genera on the larger island, and three of the same 

 species in two of the genera on the smaller one. After 

 a long period of isolation we should have approximately 

 the same number of genera and species on the two islands, 

 but they would have diverged to generic differentiation. 

 In other words, the effect of time in conjunction with iso- 

 lation is to increase the number of genera and species in 

 the family, while the index of modification undergoes little 

 change. 



