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 
of longer standing than are the adapta- 
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 bea 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 
Barriers of land, 
sea, and climate, 
