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.