8o Allen on Geographical Variation in Size among Birds. [April 



Langd, believed bj Mr. Ridgwaj (see this Bulletin, Vol. V, p. 237) to be 

 a hybrid between Helminthophila finus and Oforornis fortnosa, may be 

 counted as a third example of like character, to say nothing of the several 

 probable cases cited by Mr. Brevi^ster in the paper above referred to by 

 Mr. Townsend. — J. A. Allen.] 



NOTE ON EXCEPTIONS TO THE LAW OF INCREASE 

 IN SJZE NORTHWARD AMONG NORTH AMER- 

 ICAN BIRDS. 



BY J. A. ALLEN. 



The law of increase in size northward among North American 

 birds and mammals is so much the rule that the exceptions to it 

 are conspicuous from their rarity. In considering some years 

 since the few strongly marked examples among mammals of the 

 converse of this law I was led to formulate the following propo- 

 sitions : 



"(i) The inaxitmifn physical developmeiit of the individual 

 is attainted where the conditions of environtnent ai'e most fav- 

 orable to the life of the species. Species being primarily 

 limited in their distribution by climatic conditions, their repre- 

 sentatives living at or near either of their i-espective latitudinal 

 boundaries are more or less unfavorably affected by the influences 

 that finally limit the range of the species. ... 



"(2) The largest species of a group (genus, sub-family, or 

 family, as the case may be) are found where the group to which 

 they severally belong reaches its highest development., or where 

 it has what may be termed its ceittre of distributio7i. In other 

 words, species of a given group attain their maximum size where 

 the conditions of existence for the group in question are the most 

 favorable, just as the largest representatives of a species are found 

 where the conditions are most favorable for the existence of the 

 species. 



"(3) The most '•typical' or most geizeralized represeitta- 

 tives of a group are found also near its centre of distribtttion., 

 outlyittg forms being generally more or less ^aberrant'' or 

 specialized "* 



* " Geographical Variation among North American Mammals, especially in respect 

 to size." Bull. Geol. and Geogr. Surv. Territories, Vol. II, No. 4, July, 1876. 



