Review of Darwin’s Theory on the Origin of Species. 171 
and there is no plant so unproductive as this—and their seedlings next 
i d and so hen in twenty years there would bea 
million plants, The elephant is reckoned to be the slowest breeder of all 
own animals, and I have taken some pains to estimate its probable 
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so, at the end of the fifth century there would be alive fifteen million 
clepliants, descended from the first pair. 
“But we have better evidence on this subject than mere theoretical 
calculations, namely, the numerous recorded cases of the astonishingly 
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 man 
hich have run wild in several parts of the world; if the state- 
ments 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 
m a period of Jess than ten years. Several of the plants now 
Most Numerous over the wide plains of La Plata, clothing square leagues 
surface almost to the exclusion of all other plants, have been intro- 
al from Europe; and there are plants which now range in India, as 
m 
the enabled to breed. In such cases the geometrical ratio of increase, 
result of which never fails to be surprising, simply explains the ex- 
in th warily rapid increase and wide diffusion of naturalized productions 
it new homes.”—pp, 64, 65. 
i All plants and animals are tending to increase at a geometrical ratio ; 
Shuey most rapidly stock any station in which they could anyhow 
P a increase must be checked by destruction at some period of life.” 
. The difference between the most and the Jeast prolific species 
'SOf no account, 
&. - 
oon condor lays a couple of eggs, and the ostrich a score; and yet 
The Fulme country the condor may be the more numerous of the two. 
r 
In the world.”—p, 68. ee h 5 
. © amount of food gives the extreme limit to which each species 
{m7 increase ; but very Heasiesate it is not the obtaining of food, but the 
Pe i” Ruy to other animals, which determines the average numbers 
Pecies. “=p. 68, 
of ,vlimati plays an important part in determining the average numbers 
*pecies, and periodical seasons of extreme cold or drought, I believe 
