6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



will permit ; but in proportion as it departs from the region where the 

 optimum conditions represent the mean of a wide range of conditions, 

 it will become less and less capable of producing subtypes, for not 

 only is the range of conditions under which it is capable of existing 

 constantly narrowing, at the natural barriers becoming reduced to the 

 vanishing point, but also the time taken in its migration represents so 

 much time lost from its virile and adaptable type youth, and a corre- 

 sponding advance toward a more or less inert and inflexible type 

 maturity. 



On the borders of the range of any type, where the range of the 

 conditions under which it is possible for it to exist is very small, there 

 will be found a great number of localities where the type is able to 

 maintain itself, each of these localities differing slightly from all the 

 others, thermally or economically, or otherwise. Such thermal or 

 economic differentiation is of course also geographical. There will 

 thus result a large number of allied forms which, however, cover 

 collectively a small economic and physical range. 



An animal type intruding into a new and favorable area will at 

 once, through the opportunities of existence offered its less efficient 

 individuals, tend toward an excess of individual variation, which may 

 become extreme, until through the pressure of its own increasing 

 numbers, and the constantly increasing severity of its internal com- 

 petition, it begins to weed out the numerous less efficient varieties, 

 and to narrow them down to a very few, or even to one only, which 

 exist each within very restricted structural limits. 



Thus the existence in any area of a great number of closely allied 

 forms indicates either ( i ) the existence of a very restricted physical 

 or ecological range in which the type can maintain an existence, in 

 which case the corresponding organic varieties will be evidenced as 

 geographical forms (in the strictest sense of the term), or (2) a 

 region newly colonized, in which case a large number of more or less 

 closely related types will be found intermingled, or but partially 

 localized. 



The migratory birds offer, in the light of the preceding statements, 

 an instructive study in primary and secondary colonization. 



In the summer the temperate regions of the northern hemisphere 

 (and to a much less extent the southern) support many bird types 

 which are divided into a large number of local races, each local race 

 being a direct adaptation to a local enviroment which represents 

 economically or physically a very narrow ecological range, the sum 

 total of these narrow ecological ranges being the total range under 



