372 Grinnell, Ckesfnut-backed Chickadee. rjulv 



dees does not strike me as any greater than, for instance, between 

 Melospiza ciuerca montatia and Mclospiza cinerea rtijina, between 

 which there is continuous distribution and free interosculation. 

 But we cannot expect any two species of birds or other animals to 

 present the same degrees of differentiation in the same length of 

 time or under the same conditions, much less under different con- 

 ditions. For in no two animals is the physical organization in all 

 respects exactly the same. 



In a given aggregation of individuals constituting a new colony 

 a certain amount of time is necessary for the set of environmental 

 factors to become operative in bringing about new inheritable 

 characters to a degree perceptible to us. Then the inherited 

 effects of invasion and crossbreeding from season to season from 

 the adjacent parent center of differentiation will be evidenced less 

 and less, as time elapses, as the distance from this center increases. 

 The offspring of successively further removed unions will, of course, 

 inherit to a less and less degree the distinctive characters of the 

 ancestral stock on one side and more and more of the incipient 

 ones on the other. 



If, now, the distance is great enough to permit of the time re- 

 quired for adaptive manifestations to become innate, then we would 

 find new characters making their appearance distally nearest the 

 new center of differentiation. If the distance were too short we 

 would not find new characters showing themselves because they 

 would be constantly crowded down by the influx of the old. The 

 time factor may therefore be reduced by the intervention of an 

 impassable barrier. As an instance we find three (and there are 

 probably two other) insular forms of the Song Sparrow within a 

 limited distance among the Santa Barbara Islands, while through 

 the same distance on the adjacent mainland there is but one. Or 

 in the case of continuous distribution the time element may be 

 comparatively lessened by the great distance between the range 

 limits, and it may be still further decreased as these limits lie in 

 faunal areas of more emphatically different nature. The Horned 

 Larks as well as Song Sparrows furnish us several good examples 

 of the latter two rules. 



It is isolation, either by barriers or by sufficient distance to more 

 than counterbalance inheritance from the opposite type, that seems 



