VIII 
CAUSES OF EXTINCTION 
185 
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 1 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 extinct. If then, as appears probable, species 
first become rare and then extinct—if the too rapid increase of 
every species, even the most favoured, is steadily checked, as 
we must admit, though how and when it is hard to say—and 
if we see, without the smallest surprise, though unable to assign 
the precise reason, one species abundant and another closely- 
allied species rare in the same district—why should we feel 
such great astonishment at the rarity being carried a step 
farther to extinction ? An action going on, on every side of 
us, and yet barely appreciable, might surely be carried a little 
farther without exciting our observation. Who would feel any 
great surprise at hearing that the Megalonyx was formerly rare 
compared with the Megatherium, or that one of the fossil 
monkeys was few in number compared with one of the now 
1 See the excellent remarks on this subject by Mr. Lyell, in his Principles of 
Geology. 
