THE DISTRIBUTION OF SPECIES. 



207 



the dwarf birch and reindeer moss are the index of the 

 frigid. " Plants," says Dr. Gray, " are the thermometers 

 of the ages by which climatic extremes and climates in 

 general are best measured." In many groups anatom- 

 ical characters are not more profound or 

 Barriers of land, Qf j Qn standing than are the a dapta- 

 sea, and climate. . 



tions to heat and cold. Heat-loving ani- 

 mals are far more numerous in species than animals of 

 cold climates, though the latter often make up by greater 

 abundance of individuals. Barriers less important than 

 those of climate arise from external surroundings from 

 absence of means of defence, 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 apparently 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 ex- 

 termination. Extinction comes to those species we call 

 rare, and its advent must be unnoticed. Circumstances 

 become unfavourable to the growth or reproduction of 

 some animal. Its numbers are reduced it is rare it 

 is gone. 



The air in Indiana and Kentucky 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-de- 

 stroying, pigeon-shooting civilization has gone steadily 

 on, and now the bird which once filled our western 

 forests is 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 in- 

 crease or permanence. The dependence of the clover 

 on the number of cats in a certain neighbourhood is an 

 15 



