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 [13] 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 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 one step further to extinction?

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