lace of useless
plants, but also to battle with wintry climates, and overcome the
adverse influences of cold, sterility of soil, and other hostile
conditions of the northern zones.

One of the chief problems of biology has long been that of the
production of new varieties and species of animals as an effect of
gradual variation in structure. This is believed to be ordinarily due to
changes in the conditions of nature, animals and plants which have made
accordant changes in structure being preserved, those which have not
changed in accordance with the new conditions perishing. Where the
conditions of nature remain uniform, species may persist for long ages
unchanged, though even in the latter case changes in structure are apt
to occur, since variation in species is not wholly dependent upon
external changes. To a considerable extent it is due to causes existing
within the organism itself, fortuitous variations being occasionally
preserved when not out of harmony with the state of affairs prevailing
in the external world. Or variation may occur through the establishment
of new relations between the species inhabiting some locality while
inanimate nature remains uniform, or through migration into new
inanimate or animate surroundings. Variations, in short, may arise under
the influence of any change in the general environment which renders
necessary adaptive changes in structure. But this adaptation in some
cases takes place in the mind, new actions or methods of meeting the
contingency being adopted which render physical changes unnecessary. The
problem is a highly complicated one, and no doubt many causes have to do
with the multiplicity of effects.

There have very likely been many occasions where the changes in
structure took place rapidly, in consequence of sudden variations in
natural conditions. Such rapid changes in conditions necessarily exert a
severe stress or strain on organisms, either destroying them or causing
an equally rapid adaptation, physical or mental. In such instances it is
likely that many species perish, the change demanded being too great;
others escape by migration to better fitted localities; and others, more
mobile or less affected by the change, survive through adaptive
variations.

Of such periods of strain upon organic nature we know of only one in
recent geological times, that known as the Glacial Age, the vast
variation in climate which took place when the ice of the Far North
flowed d