OO Allen on Geographical Variation in Size among Birds. [April 
Langd, believed by Mr. Ridgway (see this Bulletin, Vol. V, p. 237) to be 
a hybrid between Helminth op hil a pinus and Oporornis formosa , may be 
counted as a third example of like character, to say nothing of the several 
probable cases cited by Mr. Brewster 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 : 
44 (1) The maximum physical development of the individual 
is attained where the conditions of environment are 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 respective 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 centre of distribution. 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 4 typical ’ or most generalized representa- 
tives of a group are found also near its centre of distribution, 
outlying forms being generally more or less 4 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. 
