No. 1.] ORGANIC VARIATION. 291 
tion than do non-migratory species ; and species which under- 
take extensive migrations, a greater amount than species 
which make migrations of less magnitude (Table XIII). This 
fact is conformable with our “law” a, since migratory species 
have as arule more extensive breeding areas than have non- 
migratory species. Thus of the 110 species which undertake 
extensive migrations, entering into the computation of table 
XIII, 104 (94.5% inhabit breeding areas of comparatively great 
extent, while but 6 (5.4%) inhabit small areas.} 
(Z2) It is the rule, that males exhibit a greater amount of 
individual variation in the dimensions than do females of the 
same species or subspecies. For of the 223 computed meas- 
urements of both sexes in Table XIX, in 131 cases (58.7%) the 
males show the greater amount of variation, and in 92 cases 
(41.2%) the females, —a difference of about 17.5% in favor of 
the males; and of the 273 measurements of both sexes com- 
puted in Table XX, in 165 cases (60.4%) the males show the 
greater amount of variation, and in 108 cases (39.5%) the 
females show the greater amount,—a difference of about 
20.8% to the advantage of the males. 
(ce) It is the rule that there is less variation in the length 
of the wing than in the length of the culmen, tarsus, or whole 
bird. This fact becomes at once apparent, by comparing the 
“curve” of variation expressed in Table XVI with the 
“‘curves’’ of variations of the other three dimensions (Tables 
DV OWT, OSV EDT): 
We may now consider the support given by these five 
“laws” to the thesis, that continuing development is always 
accompanied by variability. 
To recapitulate briefly : it follows from the data given, that 
the greatest amount of individual variation occurs, as a rule, in 
those species occupying the most extensive breeding areas ; 
that of two species occupying breeding areas approximately 
equivalent in extent, the one divided into geographical sub- 
1 These six species are Ammodramus lecontei, Spizella monticola ochracea, 
Passerella iliaca unalaschcensis, Chen caerulescens, C. hyperborea, Turdus aliciae 
bicknelli ; of these, only the first and third evince variation to the amount of 1.5% 
in two dimensions. 
