No. I.] ORGANIC VARIATION. 263 
degree, as would a non-migratory species with an equally exten- 
sive breeding area. 
(3) Extensive migration may be taken as another criterion of 
continuing development. (By the term extensive migration, I 
mean, as will be explained later, a regular periodic migration 
through a considerable distance, — 30° lat. or more.) For, as 
was shown in the preceding paragraph, a migratory species in 
wandering from its summer to its winter home, or vice versa, 
is brought into contact with a different environment, necessi- 
tating a certain amount of readaptation ; therefore, there must 
be at work a more or less continuous process of development, 
leading towards readaptations. Thus, to use a well-known 
example, a man accustomed to spend his annual holiday abroad, 
on arriving at his destination experiences the lassitude prepara- 
tory to his becoming acclimated, and experiences the same 
physical sensations on his return. Accordingly our data should 
demonstrate that species which undertake periodic migrations 
through long distances should evince a greater amount of 
variation than do stationary species, other factors being equal 
in amount. 
Other criteria of continuing development might be mentioned, 
but as the three already given are well founded and sufficient 
for the furtherance of our deductions, we shall deal but briefly 
with a fourth. If Wallace’s theory (‘ Darwinism”’) be true, 
that secondary sexual characters are most accentuated in those 
species where the sexual impulses are strongest, so that the sexual 
impulse may be considered as an important if not sole agent in 
their production, then we might consider the presence of strongly 
marked secondary sexual characters as a criterion of continuing 
development, by regarding the sexual impulse itself as a more 
or less continuing impulse to development. In other words, 
the degree of development of secondary sexual characters would 
stand in direct proportion to the continuousness and energy of 
the sexual impulse. And since secondary sexual characters are 
often different in otherwise closely allied species, being as a 
rule the least reliable (morphologically speaking) of specific dif- 
ferences, they must be regarded as of comparatively recent 
origin in each species, and thus be considered characters prob- 
