EVOLUTION AND DISTRIBUTION OF ANIMALS. 507 



than those of climate arise from external surroundings — from 

 absence of means of defense, from character of food, of air, of 

 water, and the presence of various enemies. These conditions 

 vary in their importance with each group of animals, yet appar- 

 ently the least of them may be able to limit the range of species. 

 To limit the range is the first step toward extinction, for to cease 

 to advance is to retreat. Adverse conditions may invade even 

 the heart of its distribution, causing reduction of numbers, which, 

 if long continued, must mean rarity and final extermination. 

 Extinction comes to those species we call rare, and its advent 

 must be unnoticed. Circumstances become unfavorable to the 

 growth or reproduction of some animal. Its numbers are reduced 

 — it is rare — it is gone. 



The air in Indiana but a few years since was dark with the 

 hordes of passenger pigeons at the time of their fall migrations. 

 The advance of a tree-destroying, pigeon-shooting civilization has 

 gone steadily on, and now who has seen a passenger pigeon ? I 

 have seen them, and I have a skin or two in my collection, but 

 the bird I knew as filling the trees in my boyhood is now in the 

 same region an ornithological curiosity. 



A very slight change in the environment of any species may 

 be a matter of the greatest moment as regards its increase or per- 

 manence. The dependence of the clover on the number of cats in 

 a certain neighborhood is an illustration given us by Mr. Darwin. 

 The clover depends on the bumble-bee for the fertilization of its 

 pods. The nests of the bumble-bee are destroyed by the field- 

 mouse, which is thus an enemy of the clover. The balance is 

 restored by the work of the cat, who captures the mouse and pre- 

 vents its ravages on the nests of the bee. The old nursery jingle 

 of the cow that tossed the dog that worried the cat that killed the 

 rat is repeated throughout nature. With any change in any of 

 the elements in this series the whole equilibrium of nature is 

 interrupted. For this equilibrium is apparent only — a sort of 

 armed neutrality, an established order of things which the super- 

 ficial observer mistakes for real peace and permanence. 



In some groups we find evidence of a progressive adaptation of 

 individuals to circumstances — for example, to climate, ending in 

 the formation of new species to accord with changed conditions 

 of temperature. We may illustrate this by means of the arctic 

 birches. In Norway, as in most northern regions with a moist 

 climate, there are large forests of birches. In the valleys, where 

 the summers are warm and reasonably long, the birches of differ- 

 ent species grow to be considerable trees. Farther to the north, 

 or higher up the mountains, the summer is too short for the 

 growth of birch-trees, and their place is taken by birches which 

 never pass beyond the size of small bushes. Still higher up there 



