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? 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 would feel any great surprise at hearing that the Magalonyx 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 to death -- to feel no surprise at sickness -- but when the sick man dies to wonder, and to believe that he died through violence.
[1] Mr. Waterhouse has drawn up a detailed description of this head, which I hope he will publish in some Journal.
[2] A nearly similar abnormal, but I do not know whether hereditary, structure has been observed in the carp, and likewise in the crocodile of the Ganges: Histoire des Anomalies, par M. Isid. Geoffroy St. Hilaire, tom. i. p. 244.
[3] M. A. d'Orbigny has given nearly a similar account of these dogs, tom. i. p. 175.
[4] I must express my obligations to Mr. Keane, at whose house I was staying on the Berquelo, and to Mr. Lumb at Buenos Ayres, for without their assistance these valuable remains would never have reached England.
[5] Lyell's Principles of Geology, vol. iii. p. 63.
[6] The flies which frequently accompany a ship for some days on its passage from harbour to harbour, wandering from the vessel, are soon lost, and all disappear.
[7] Mr. Blackwall, in his Researches in Zoology, has many excellent observations on the habits of spiders.
[8] An abstract is given in No. IV. of the Magazine of Zoology and Botany.
[9] I found here a species of cactus, described by Professor Henslow, under the name of Opuntia Darwinii (Magazine of Zoology and Botany, vol. i. p. 466), which was remarkable for the irritability of the stamens, when I inserted either a piece of stick or the end of my finger in the flower. The segments of the perianth also closed on the pistil, but more slowly than the stamens. Plants of this family, generally considered as tropical, occur in North America (Lewis and Clarke's Travels, p. 221), in the same high latitude as here, namely, in both cases, in 47 degs.
[10] These insects were not uncommon beneath stones. I found one cannibal scorpion quietly devouring another.
[11] Shelley, Lines on Mt. Blanc.
[12] I have lately heard that Capt. Sulivan, R.N., has found numerous fossil bones, embedded in regular strata, on the banks of the R. Gallegos, in lat. 51 degs. 4'. Some of the bones are large; others are small, and appear to have belonged to an armadillo. This is a most interesting and important discovery.
[13] See the excellent remarks on this subject by Mr. Lyell, in his Principles of Geology.