3ï8 
a very close and definite correlation between the whole 
organism and its surrounding conditions ; and hence con 
siderable changes—and, indeed, even slight modifications— 
in the conditions may be fatal to the species in proportion 
to its adaptation. 
A species may become extinct in certain cases, not 
because it is not eminently adapted to the position in 
which it is placed, in a great many respects, but because 
some very slight disability may prevent it from flourishing 
or may cause it to become altogether extinct. In his 
Origin of Species, Mr. Darwin propounds the problem 
how the horse came to be exterminated in South America. 
He says :— 
“ Seeing that the horse, since its introduction by the Spaniards into 
South America, has run wild over the whole country, and has 
increased in numbers at an unparalleled rate, I asked myself what 
could so recently have exterminated the former horse under con 
ditions of life apparently so favourable.”—(Origin of Species, p. 2Ç4.) 
The solution of this problem may perhaps be found in a 
passage in the same work, where it is asserted that, in 
Paraguay,— 
“ Neither cattle nor horses nor dogs have ever run wild, though 
they swarm southward and northward in a feral state ; and Azara 
and Rengger have shown that this is caused by the greater number 
in Paraguay of a certain fly, which lays its eggs in the navels of 
these animals when first born.”—(Origin of Species, p. 36.) 
In this case extinction follows, because among many 
favourable conditions there is one which is fatal to the 
welfare of the species. 
Sometimes two species are found which could flourish 
equally well apart from one another, and yet one species 
seems to have the power to exterminate the other when 
brought into juxtaposition.