sexes of the same species."

The influence of changes of climate on variation has been studied to
especial advantage in North America, owing to its great extent, and to
the fact that its territory ranges from the polar to the tropical
regions, and from the Atlantic to the Pacific Ocean. As respects
climatic variation in birds, Professor Baird first took up the inquiry,
which was greatly extended, with especial relation to the formation of
local varieties, by Dr. J. A. Allen,[228] who was the first to ascertain
by careful measurements, and by a study of the difference in plumage and
pelage of individuals inhabiting distant portions of a common habitat,
the variations due to climatic and local causes.

"That varieties," he says, "may and do arise by the action of climatic
influences, and pass on to become species; and that species become, in
like manner, differentiated into genera, is abundantly indicated by the
facts of geographical distribution, and the obvious relation of local
forms to the conditions of environment. The present more or less
unstable condition of the circumstances surrounding organic beings,
together with the known mutations of climate our planet has undergone in
past geological ages, point clearly to the agency of physical conditions
as one of the chief factors in the evolution of new forms of life. So
long as the environing conditions remain stable, just so long will
permanency of character be maintained; but let changes occur, however
gradual or minute, and differentiations begin." He inclines to regard
the modifications as due rather to the direct action of the conditions
of environment than to "the round-about process of natural selection."
He also admits that change of habits and food, use and disuse, are
factors.

The same kind of inquiry, though on far less complete data, was extended
by the present writer[229] in 1873 to the moths, careful measurements of
twenty-five species of geometrid moths common to the Atlantic and
Pacific coasts of North America showing that there is an increase in
size and variation in shape of the wings, and in some cases in color, in
the Pacific Coast over Eastern or Atlantic Coast individuals of the same
species, the differences being attributed to the action of climatic
causes. The same law holds good in the few Notodontian moths common to
both sides of our continent. Similar studies, the results depending on
careful measurements of many individuals, have recentl