ich
has existed for immemorial years, we find the tiger, his stripes
simulating jungle reeds, his noiseless approach learnt from nature in
countless millions of lessons of success and failure, his perfectly
powerful claws and execution methods; and, living in the same jungle,
and with him as one of the conditions of life, are small deer, alert,
swift, light of build, inconspicuous of colour, sharp of hearing,
keen-eyed, keen-scented-- because any downward variation from
these attributes means swift and certain death. To capture the deer is
a condition, of the tiger's life, to escape the tiger a condition of the
deer's; and they play a great contest under these conditions, with life
as the stake. The most alert deer almost always escape; the least
so, perish.

Section 51. But conditions may alter. For instance, while most of
these deer still live in the jungle with tigers, over a considerable area
of their habitat, some change may be at work that thins the jungle,
destroys the tigers in it, and brings in, let us say, wolves, as an
enemy to the deer, instead of tigers. Now, against the wolves, which
do not creep, but hunt noisily, and which do not spring suddenly upon
prey, but follow by scent, and run it down in packs, keen eyes, sharp
ears, acute perceptions, will be far less important than endurance in
running. The deer, under the new conditions, will need coarser and
more powerful limbs, and a larger chest; it will be an advantage to be
rough and big, instead, of frail and inconspicuous, and the ears and
eyes need not be so large. The old refinements will mean weakness
and death; any variation along the line of size and coarseness will be
advantageous. Slight and delicate deer will be continually being killed,
rougher and stronger deer continually escaping. And so gradually,
under the new circumstances, if they are not sufficient to exterminate
the species, the finer characteristics will be eliminated, and a new
variety of our old jungle deer will arise, and, if the separation and
contrast of the conditions is sufficiently great and permanent, we
may, at last, in the course of ages, get a new kind of deer specifically
different in its limbs, body, sense organs, colour, and instincts, from
the deer that live in the jungle. And these latter will, on their side, be
still continually more perfected to the jungle life they are leading.

Section 52. Take a wider range of time and vaster changes of
condition than this,