180 
closely allied iii habits, being rare and the other abundant in the 
same district It asked how this is, one immediately 
lejjlies that it is determined by some slight difference in climate, 
food, or the number of enemies ; yet how rarely, if ever, we can 
point out the precise cause and manner of action of the check ! 
AVe are, therefore, driven to the conclusion, that some causes, 
generally quite inappreciable by us, determine Avhether a given 
species shall be abundant or scanty in numbers.” This process of 
extinction, although going on day by day under our very eyes, is so 
stealthy in its action that species imperceptibly die out almost 
unheeded, till at last we wake iqi to the startling fact that another 
blank has arisen; something has gone down in the struggle for 
existence, and given place to some other form more adapted— 
although in what precise way ive may not clearly understand— to 
the changed circumstances. How astonishing it seems that the 
Horse should have become extinct in America late in geologic 
times, and yet the country still remain so jieculiarly adapted to this 
cieature, that when re-introduced it should ra^iidly increase in 
numbers, so rapidly indeed as in some instances to swarm in 
countless herds.* AVlierein does the introduced Horse of the 
lAimpas differ from its fossil predecessor 1 and ivliat can have led 
to the extinction of the latter in a country in every way so suitable 
to the rc(iuirements of the certainly closely allied European 
species ? t 
* Mr. Darwin says that the Horse was first landed at Buenos Ayres in 
1537, and the colony being then for a time deserted the Horse ran wild ; in 
1580, only forty years afterwards, it was met with at the Strait of Magellan. 
t Since the above was written Wallace’s ‘Island Life’ has appeared; 
the following passage at p. 210 of which is so instructive, that I need not 
apologize for cpioting it. It seems quite possible that some sucli insect as is 
there mentioned might produce the extinction of any particular animal to 
which it was attached, itself becoming extinct with the proper receptacle for its 
larval condition. Free from such an enemy, the animal, when re-introduced, 
might lapidly increase in a countiy, but for the insect plague now absent, 
admirably suited to its requirements. Mr. Wallace savs; — “The complex 
nianner in which animals are related to each other is well exhibited by the 
importance of insects which in many parts of the world limit the numbers 
01 determine the very existence of some of the higher animals. Mr. 
Darwin says : ‘ Perhaps Paraguay offers the most curious instance of this ; 
foi here neither cattle, nor Horses, nor Dogs have ever run wild, though they 
