Survival of the Fittest. 429 
or with the conditions of the environment. This is the 
doctrine of Malthus applied with manifold force to the entire 
vegetable and animal kingdoms. Although some species 
may be now increasing at a very high rate in numbers, yet 
all cannot do so, for the earth would not be able to contain 
them. Slow-breeding man has doubled in twenty-five years, 
and should he go on at this rate for a few thousand years, 
there would literally not be standing room for his progeny. 
It has been calculated that, if an annual plant produced only 
two seeds, and their seedlings next year produced two, and 
the same rate of increase was kept up for twenty years, there 
would be a million of plants as the result. Even the elephant, 
which is reckoned the slowest breeder of all known animals, 
would after a period of from seven hundred and forty to seven 
hundred and fifty years leave nearly nineteen million ele- 
phants as descendants from the first pair. 
Much better evidence than mere theoretical calculations 
are not wanting on this subject. Instances are recorded of 
the astonishingly rapid increase of various animals in a state 
of nature, when conditions have been favorable to them, 
during two or three succeeding seasons. More striking, 
however, is the evidence from domestic animals that have 
run wild in several parts of the world. Were not the state- 
ments of the rate of increase of cattle and horses in South 
America, and latterly in Australia, where millions now 
abound, well authenticated, they would have been incredible. 
Cases could be mentioned of introduced plants that have 
become quite common throughout entire islands in a period 
of less than twelve years. Several of these plants, the car- 
doon and a rare thistle, which were introduced from Europe, 
clothe square leagues of the surface of the wide plains of 
the La Plata almost to the exclusion of all other plants; 
and there are plants which now range in India, from Cape 
Comorin to the Himalaya, which have been imported 
from America since its discovery. In all such cases, and 
endless instances could be adduced, no intelligent person 
