ndividual. The affected region has become unsuitable,
cheerless, uncomfortable; they abandon it; and as the boundary they
thus, in the one case, can not, and in the other will not overpass,
advances, so do they recede before it. If the change were abrupt, or
took place by a sudden crisis, there would seem to be no other possible
event than an overcrowding of the unaffected region and a desolation of
the part that had varied. But, since a developing cell under a new
condition produces a new form, and since the physical change is taking
place with extreme slowness, the appearance of modified structures
ensues. And thus, by decline of temperature, two distinct results are
accomplished--first the production of organic forms in an order of
succession, new ones replacing the old, as if they were transmutations
of them, and, secondly, geographical distribution.

[Sidenote: Delusive nature of organic equilibrium.] In my "Physiology" I
have endeavoured to explain in detail the principles here set forth. I
have endeavoured to show that the aspect of sameness presented by an
animal or plant is no proof of unchangeability. Those forms retain in
our times their special aspect because the conditions of the theatre in
which they live do not change; but let the mean temperature rise, let
the sun-rays become brighter, change the composition of the air, and
forthwith the world of organization would show how profoundly it was
affected. Nor need such changes, in one sense, be more than
insignificant to produce prodigious results. Thus the air contains only
1/2000 of its volume of carbonic acid gas. That apparently trifling
quantity taken away, in an instant the whole surface of the earth would
become a desolate waste, without the possibility of vegetable life.

[Sidenote: The Coal period.] As physical geology advanced, the Coal
period was perceived to be the chief epoch in the history of our planet.
Through a slow decline of temperature, a possibility had gradually been
attained, so far as the condition of heat was concerned, for a luxuriant
vegetable growth. All that prodigious mass of carbon now found in the
earth in the various forms of coal existed as carbonic acid in the
atmosphere. The proportion of free oxygen was less than at present by a
volume equal to the excess of carbonic acid. [Sidenote: Effects of light
on the atmosphere,] A change in the constitution of this primaeval
atmosphere was occasioned by the action of the light; fo