1834.] CAUSES OF EXTINCTION, 175 
view, it will appear less perplexing. We do notsteadily bear in 
mind, how profoundly ignorant we are of the conditions of exist- 
ence of every animal; nor do we always remember, that some 
check is constantly preventing the too rapid increase of every 
organized being left in a state of nature. The supply of food, on 
an average, remains constant ; yet the tendency in every animal to 
increase by propagation is geometrical ; and its surprising effects 
have nowhere been more astonishingly shown, than in the case 
of the European animals run wild during the last few centuries 
in America. Every animal inastate of nature regularly breeds ; 
yet in a species long established, any great increase in numbers is 
obviously impossible, and must be checked by some means. We 
are, nevertheless, seldom able with certainty to tell in any given 
species, at what period of life, or at what period of the year, or 
whether only at long intervals, the check falls; or, again, what 
is the precise nature of the check. Hence probably it is, that 
we feel so little surprise at one, of two species closely allied in 
habits, being rare and the other abundant in the same district ; 
or, again, that one should be abundant in one district, and 
another, filling the same place in the economy of nature, should 
be abundant in a neighbouring district, differing very little in its 
conditions. ‘If asked how this is, one immediately replies 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! We are, 
therefore, driven to the conclusion, that causes generally quite 
inappreciable by us, determine whether a given species shall be 
abundant or scanty in numbers. 
In the cases where we can trace the extinction of a species 
through man, either wholly or in one limited district, we know 
that it becomes rarer and rarer, and is then lost: it would be 
difficult to point out any just distinction * between a species 
destroyed by man or by the increase of its natural enemies. The 
evidence of rarity preceding extinction, is more striking in the 
successive tertiary strata, as remarked by several able observers ; 
it has often been found that a shell very common in a tertiary 
stratum is now most rare, and has even long been thought to be 
* See the excellent remarks on this subject by Mr. Lyell, in his Prin- 
ciples of Geology. ‘ 
