rapid increase of various animals in a state of nature, when circumstances
have been favorable to them during two or three following seasons. Still
more striking is the evidence from our domestic animals of many kinds which
have run wild in several parts of the world; if the statements of the rate
of increase of slow-breeding cattle and horses in South America, and
latterly in Australia, had not been well authenticated, they would have
been quite incredible. So it is with plants: cases could be given of
introduced plants which have become common throughout whole islands in a
period of less than ten years. Several of the plants now most numerous over
the wide plains of La Plata, clothing square leagues of surface almost to
the exclusion of all other plants, have been introduced from Europe; and
there are plants which now range in India, as I hear from Dr. Falconer,
from Cape Comorin to the Himalaya, which have been imported from America
since its discovery. In such cases, and endless instances could be given,
no one supposes that the fertility of these animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious
explanation is, that the conditions of life have been very favorable, and
that there has consequently been less destruction of the old and young, and
that nearly all the young have been enabled to breed. In such cases the
geometrical ratio of increase, the result of which never fails to be
surprising, simply explains the extraordinarily rapid increase and wide
diffusion of naturalized productions in their new homes."--(pp. 64, 65.)

"All plants and animals are tending to increase at a geometrical ratio; all
would most rapidly stock any station in which they could anyhow exist; the
increase must be checked by destruction at some period of life."--(p. 65.)


The difference between the most and the least prolific species is of no
account: