626 General Notes. [ October, 
it is almost nil. Contrary to the general supposition, the variation in 
size among representatives of the same species is not always a decrease 
with the decrease of the latitude of the locality, but is in some cases ex- 
actly the reverse, in some species there being a very considerable and in- 
disputable increase southward. This, for instance, is very markedly true 
of some species of Felis and in Procyon lotor. Consequently, the very 
generally received impression that in North America the species of Mam- 
malia diminish in size southward, or with the decrease in the latitude 
(and altitude) of the locality, requires modification. While such is gen- 
erally the case, the reverse of this too often occurs, with occasional in- 
stances also of a total absence of variation in size with locality, to be 
considered as forming “the exceptions” necessary to “ prove the rule.” 
That there are such exceptions, among both birds and mammals, I 
have been long aware, and long since noticed that where there is an act- 
ual increase in size to the southward it occurs in species that belong to 
families or genera that are mainly developed within the tropics, there 
reaching their maximum development, both in respect to the number 0 
their specific representatives and in respect to the size to which some of 
the species attain. This fact seems also to have been observed by 
others : 
Most of the mammals of North America belong to families, subfam- 
ilies, or genera which have their greatest development in the temperate 
or colder portions of the northern hemisphere, as the Cervide, the Can- 
ide, the Mustelide, the Sciuride (especially the subfamily Arctomyin@); 
the Leporide, the Oastoride, the Arvicoline among the Muride, the 
Saccomyide, Geomyide, etc. These rarely present an exception to the 
riation is less 
(in fact, occasionally almost nil) in some species than in the others. 
The more marked exceptions, or those in which there is an actual in- 
crease in size southward, occur in those families that reach their highest 
development within the tropics, as the Felide and Procyonide. 
In some species (as I have elsewhere noticed) there probably oxtail 3 
double decadence in size, the individual reaching its maximum dimen- 
sions where the conditions of environment are most favorable for the ex- 
istence of the species, and diminishing in size toward t 
scarcity of food and severity of climate) as well as toward the southern 
(in consequence of the enervating influence of tropical or semitropic4 
conditions) limit of its distribution. | 
1 I find that Mr. Robert Ridgway, some two years since, thus referred to this pont 
In alluding to the smaller size of Mexican specimens of Catherpes Mexicanus ner 
pared with specimens from Colorado (O. Mezxicanus var. conspersus), he 539%, 
we find this peculiarity exactly paralleled in the 7) hryothorus ludovicianus of 
lantic States, may not these facts point out a law to the effect i -n ofthe 
‘ ‘tude is toward the region of ah 
highest development of the group?” ( Baird, Brewer, and Ridgway’s Birds of No 
America, vol. iii., App., p. 503, 1874.) 
he northern (through 
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