"We do not steadily bear in mind," remarks Mr Darwin, "how profoundly ignorant we are of the condition of existence of every animal; nor do we always remember that some check is constantly preventing the too rapid increase of every organised 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 shewn, than in the case of the European animals run wild during the last few centuries in America. Every animal in a state 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 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 further to extinction? An action going on, on every side of us, and yet barely appreciable, might surely be carried a little further, without exciting our observation. Who could 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 living Monkeys? and yet, in this comparative rarity, we should have the plainest evidence of less favourable conditions for their existence. To admit that species generally become rare before they become extinct—to feel no surprise at the comparative rarity of one species with another, and yet to call in some extraordinary agent and to marvel greatly when a species ceases to exist, appears to me much the same as to admit that sickness in the individual is the prelude of death—to feel no surprise at sickness—but when the sick man dies to wonder, and to believe that he died through violence."[69]
Geographical distribution is an important element in this question of extinction. A species that is spread over a wide region is far more likely to survive than one which is confined to a limited district; and extraneous influences acting prejudicially will exterminate a species which is confined to an island much sooner than if it had a continent to retire upon. We have seen how the Nestor Parrot became extinct in New Zealand, while it survived in Norfolk Island, because the former was colonised by the Maori race, while the latter remained in its virginity. But how quickly did the poor Parrot succumb as soon as man set his foot on Norfolk and Philip Islands! And how brief was the lease of life accorded to the Didunculus, when once the "Pussies" found their way to the little Samoa isles!
Very many islands have a fauna that is to a great extent peculiar to themselves. I know that, in Jamaica, the Humming-birds, some of the Parrots, some of the Cuckoos, most of the Pigeons, many of the smaller birds, and, I think, all of the Reptiles, are found nowhere else. Nay, more, that even the smaller islands of the Antilles have each a fauna of its own, unshared with any other land;—its own Humming-birds, its own Lizards and Snakes; its own Butterflies and Beetles, its own Spiders, its own Snails, its own Worms. How likely are some of these very limited species to become extinguished! By the increasing aggressions of clearing and cultivating man; by slight changes of level; even by electric and meteoric phenomena acting very locally. I find that, in Jamaica, many of the animals peculiar to the island are not spread over the whole surface, limited as that is, but are confined to a single small district. In some cases, the individuals are but few, even in that favoured locality; how easily we may conceive of a season drier than ordinary, or wetter than ordinary, or a flood, or a hurricane of unusual violence, or a volcanic eruption, either killing outright these few individuals, or destroying their means of living, and so indirectly destroying them by starvation. And then the species has disappeared!
The common Red Grouse, so abundantly seen during the season hanging at every poulterer's and game-dealer's shop in London, is absolutely unknown out of the British Isles. It could not live except in wide, unenclosed, uncultivated districts; so that when the period arrives that the whole of British land is enclosed and brought under cultivation, the Grouse's lease of life will expire. We owe it to our hard-worked members of Parliament to hope that this condition of things may be distant.
II.
THE MARVELLOUS.
The vulgar mind is very prone to love the marvellous, and to count for a prodigy every unusual phenomenon, every occurrence not perfectly accountable on any hypothesis which is familiar to them. The poetical period of history in every country is full of prodigies; for in the dawn of civilisation the physical laws of nature are little understood, and multitudes of natural phenomena are either referred to false causes, or, being unreferrible to any recognised cause, are set down as mere wonders. It is the province of science to dispel these delusions, to expose the undiscovered, but by no means undiscoverable, origins of unusual events, and thus to be continually narrowing the limits of the unknown. These limits, however, have not even yet quite reached the minuteness of a mathematical point; and there are a few marvels left for the indefatigable rummagings of modern science to explain.