{323} See _More Letters_, vol. I. pp. 344-7, for Darwin"s interest in the celebrated observations of Hilgendorf and Hyatt.
{324} This corresponds partly to _Origin_, Ed. i. p. 294, vi. p.
431.
{325} _Origin_, Ed. i. p. 299, vi. p. 437.
CHAPTER V
GRADUAL APPEARANCE AND DISAPPEARANCE OF SPECIES{326}
{326} This chapter corresponds to ch. X of _Origin_, Ed. i., vi.
ch. XI, "On the geological succession of organic beings."
In the Tertiary system, in the last uplifted beds, we find all the species recent and living in the immediate vicinity; in rather older beds we find only recent species, but some not living in the immediate vicinity{327}; we then find beds with two or three or a few more extinct or very rare species; then considerably more extinct species, but with gaps in the regular increase; and finally we have beds with only two or three or not one living species. Most geologists believe that the gaps in the percentage, that is the sudden increments, in the number of the extinct species in the stages of the Tertiary system are due to the imperfection of the geological record. Hence we are led to believe that the species in the Tertiary system have been gradually introduced; and from a.n.a.logy to carry on the same view to the Secondary formations. In these latter, however, entire groups of species generally come in abruptly; but this would naturally result, if, as argued in the foregoing chapter, these Secondary deposits are separated by wide epochs. Moreover it is important to observe that, with our increase of knowledge, the gaps between the older formations become fewer and smaller; geologists of a few years standing remember how beautifully has the Devonian system{328} come in between the Carboniferous and Silurian formations. I need hardly observe that the slow and gradual appearance of new forms follows from our theory, for to form a new species, an old one must not only be plastic in its organization, becoming so probably from changes in the conditions of its existence, but a place in the natural economy of the district must [be made,] come to exist, for the selection of some new modification of its structure, better fitted to the surrounding conditions than are the other individuals of the same or other species{329}.
{327} _Origin_, Ed. i. p. 312, vi. p. 453.
{328} In the margin the author has written "Lonsdale." This refers to W. Lonsdale"s paper "Notes on the age of the Limestone of South Devonshire," _Geolog. Soc. Trans._, Series 2, vol. V. 1840, p. 721.
According to Mr H. B. Woodward (_History of the Geological Society of London_, 1907, p. 107) "Lonsdale"s "important and original suggestion of the existence of an intermediary type of Palaeozoic fossils, since called Devonian," led to a change which was then "the greatest ever made at one time in the cla.s.sification of our English formations"." Mr Woodward"s quotations are from Murchison and Buckland.
{329} Better begin with this. If species really, after catastrophes, created in showers over world, my theory false. In the Tertiary system the same facts, which make us admit as probable that new species have slowly appeared, lead to the admission that old ones have slowly disappeared, not several together, but one after another; and by a.n.a.logy one is induced to extend this belief to the Secondary and Palaeozoic epochs. In some cases, as the subsidence of a flat country, or the breaking or the joining of an isthmus, and the sudden inroad of many new and destructive species, extinction might be locally sudden. The view entertained by many geologists, that each fauna of each Secondary epoch has been suddenly destroyed over the whole world, so that no succession could be left for the production of new forms, is subversive of my theory, but I see no grounds whatever to admit such a view. On the contrary, the law, which has been made out, with reference to distinct epochs, by independent observers, namely, that the wider the geographical range of a species the longer is its duration in time, seems entirely opposed to any universal extermination{330}. The fact of species of mammiferous animals and fish being renewed at a quicker rate than mollusca, though both aquatic; and of these the terrestrial genera being renewed quicker than the marine; and the marine mollusca being again renewed quicker than the Infusorial animalcula, all seem to show that the extinction and renewal of species does not depend on general catastrophes, but on the particular relations of the several cla.s.ses to the conditions to which they are exposed{331}.
{330} Opposite to this pa.s.sage the author has written "d"Archiac, Forbes, Lyell."
{331} This pa.s.sage, for which the author gives as authorities the names of Lyell, Forbes and Ehrenberg, corresponds in part to the discussion beginning on p. 313 of _Origin_, Ed. i., vi. p. 454.
Some authors seem to consider the fact of a few species having survived{332} amidst a number of extinct forms (as is the case with a tortoise and a crocodile out of the vast number of extinct sub-Himalayan fossils) as strongly opposed to the view of species being mutable. No doubt this would be the case, if it were presupposed with Lamarck that there was some inherent tendency to change and development in all species, for which supposition I see no evidence. As we see some species at present adapted to a wide range of conditions, so we may suppose that such species would survive unchanged and unexterminated for a long time; time generally being from geological causes a correlative of changing conditions. How at present one species becomes adapted to a wide range, and another species to a restricted range of conditions, is of difficult explanation.
{332} The author gives Falconer as his authority: see _Origin_, Ed.
i. p. 313, vi. p. 454.
_Extinction of species._
The extinction of the larger quadrupeds, of which we imagine we better know the conditions of existence, has been thought little less wonderful than the appearance of new species; and has, I think, chiefly led to the belief of universal catastrophes. When considering the wonderful disappearance within a late period, whilst recent sh.e.l.ls were living, of the numerous great and small mammifers of S. America, one is strongly induced to join with the catastrophists. I believe, however, that very erroneous views are held on this subject. As far as is historically known, the disappearance of species from any one country has been slow--the species becoming rarer and rarer, locally extinct, and finally lost{333}. It may be objected that this has been effected by man"s direct agency, or by his indirect agency in altering the state of the country; in this latter case, however, it would be difficult to draw any just distinction between his agency and natural agencies. But we now know in the later Tertiary deposits, that sh.e.l.ls become rarer and rarer in the successive beds, and finally disappear: it has happened, also, that sh.e.l.ls common in a fossil state, and thought to have been extinct, have been found to be still living species, but very _rare_ ones{334}.
If the rule is that organisms become extinct by becoming rarer and rarer, we ought not to view their extinction, even in the case of the larger quadrupeds, as anything wonderful and out of the common course of events. For no naturalist thinks it wonderful that one species of a genus should be rare and another abundant, notwithstanding he be quite incapable of explaining the causes of the comparative rareness{335}. Why is one species of willow-wren or hawk or woodp.e.c.k.e.r common in England, and another extremely rare: why at the Cape of Good Hope is one species of rhinoceros or antelope far more abundant than other species? Why again is the same species much more abundant in one district of a country than in another district? No doubt there are in each case good causes: but they are unknown and unperceived by us. May we not then safely infer that as certain causes are acting _unperceived_ around us, and are making one species to be common and another exceedingly rare, that they might equally well cause the final extinction of some species without being perceived by us? We should always bear in mind that there is a recurrent struggle for life in every organism, and that in every country a destroying agency is always counteracting the geometrical tendency to increase in every species; and yet without our being able to tell with certainty at what period of life, or at what period of the year, the destruction falls the heaviest. Ought we then to expect to trace the steps by which this destroying power, always at work and scarcely perceived by us, becomes increased, and yet if it continues to increase ever so slowly (without the fertility of the species in question be likewise increased) the average number of the individuals of that species must decrease, and become finally lost. I may give a single instance of a check causing local extermination which might long have escaped discovery{336}; the horse, though swarming in a wild state in La Plata, and likewise under apparently the most unfavourable conditions in the scorched and alternately flooded plains of Caraccas, will not in a wild state extend beyond a certain degree of lat.i.tude into the intermediate country of Paraguay; this is owing to a certain fly depositing its eggs on the navels of the foals: as, however, man with a _little_ care can rear horses in a tame state _abundantly_ in Paraguay, the problem of its extinction is probably complicated by the greater exposure of the wild horse to occasional famine from the droughts, to the attacks of the jaguar and other such evils. In the Falkland Islands the check to the _increase_ of the wild horse is said to be loss of the sucking foals{337}, from the stallions compelling the mares to travel across bogs and rocks in search of food: if the pasture on these islands decreased a little, the horse, perhaps, would cease to exist in a wild state, not from the absolute want of food, but from the impatience of the stallions urging the mares to travel whilst the foals were too young.
{333} This corresponds approximately to _Origin_, Ed. i. p. 317, vi. p. 458.
{334} The case of _Trigonia_, a great Secondary genus of sh.e.l.ls surviving in a single species in the Australian seas, is given as an example in the _Origin_, Ed. i. p. 321, vi. p. 463.
{335} This point, on which the author laid much stress, is discussed in the _Origin_, Ed. i. p. 319, vi. p. 461.
{336} _Origin_, Ed. i. p. 72, vi. p. 89.
{337} This case does not occur in the _Origin_, Ed.
From our more intimate acquaintance with domestic animals, we cannot conceive their extinction without some glaring agency; we forget that they would undoubtedly in a state of nature (where other animals are ready to fill up their place) be acted on in some part of their lives by a destroying agency, keeping their numbers on an average constant. If the common ox was known only as a wild S. African species, we should feel no surprise at hearing that it was a very rare species; and this rarity would be a stage towards its extinction. Even in man, so infinitely better known than any other inhabitant of this world, how impossible it has been found, without statistical calculations, to judge of the proportions of births and deaths, of the duration of life, and of the increase and decrease of population; and still less of the causes of such changes: and yet, as has so often been repeated, decrease in numbers or rarity seems to be the high-road to extinction. To marvel at the extermination of a species appears to me to be the same thing as to know that illness is the road to death,--to look at illness as an ordinary event, nevertheless to conclude, when the sick man dies, that his death has been caused by some unknown and violent agency{338}.
{338} An almost identical sentence occurs in the _Origin_, Ed. i.
p. 320, vi. p. 462.
In a future part of this work we shall show that, as a general rule, groups of allied species{339} gradually appear and disappear, one after the other, on the face of the earth, like the individuals of the same species: and we shall then endeavour to show the probable cause of this remarkable fact.
{339} _Origin_, Ed. i. p. 316, vi. p. 457.
CHAPTER VI
ON THE GEOGRAPHICAL DISTRIBUTION OF ORGANIC BEINGS IN PAST AND PRESENT TIMES
For convenience sake I shall divide this chapter into three sections{340}. In the first place I shall endeavour to state the laws of the distribution of existing beings, as far as our present object is concerned; in the second, that of extinct; and in the third section I shall consider how far these laws accord with the theory of allied species having a common descent.
{340} Chapters XI and XII in the _Origin_, Ed. i., vi. chs. XII and XIII ("On geographical distribution") show signs of having been originally one, in the fact that one summary serves for both. The geological element is not separately treated there, nor is there a separate section on "how far these laws accord with the theory, &c."
In the MS. the author has here written in the margin "If same species appear at two spot at once, fatal to my theory." See _Origin_, Ed. i. p. 352, vi. p. 499
SECTION FIRST.
_Distribution of the inhabitants in the different continents._
In the following discussion I shall chiefly refer to terrestrial mammifers, inasmuch as they are better known; their differences in different countries, strongly marked; and especially as the necessary means of their transport are more evident, and confusion, from the accidental conveyance by man of a species from one district to another district, is less likely to arise. It is known that all mammifers (as well as all other organisms) are united in one great system; but that the different species, genera, or families of the same order inhabit different quarters of the globe. If we divide the land{341} into two divisions, according to the amount of difference, and disregarding the numbers of the terrestrial mammifers inhabiting them, we shall have first Australia including New Guinea; and secondly the rest of the world: if we make a three-fold division, we shall have Australia, S.
America, and the rest of the world; I must observe that North America is in some respects neutral land, from possessing some S. American forms, but I believe it is more closely allied (as it certainly is in its birds, plants and sh.e.l.ls) with Europe. If our division had been four-fold, we should have had Australia, S. America, Madagascar (though inhabited by few mammifers) and the remaining land: if five-fold, Africa, especially the southern eastern parts, would have to be separated from the remainder of the world. These differences in the mammiferous inhabitants of the several main divisions of the globe cannot, it is well known, be explained by corresponding differences in their conditions{342}; how similar are parts of tropical America and Africa; and accordingly we find some _a.n.a.logous_ resemblances,--thus both have monkeys, both large feline animals, both large Lepidoptera, and large dung-feeding beetles; both have palms and epiphytes; and yet the essential difference between their productions is as great as between those of the arid plains of the Cape of Good Hope and the gra.s.s-covered savannahs of La Plata{343}. Consider the distribution of the Marsupialia, which are eminently characteristic of Australia, and in a lesser degree of S. America; when we reflect that animals of this division, feeding both on animal and vegetable matter, frequent the dry open or wooded plains and mountains of Australia, the humid impenetrable forests of New Guinea and Brazil; the dry rocky mountains of Chile, and the gra.s.sy plains of Banda Oriental, we must look to some other cause, than the nature of the country, for their absence in Africa and other quarters of the world.
{341} This division of the land into regions does not occur in the _Origin_, Ed. i.
{342} _Origin_, Ed. i. p. 346, vi. p. 493.
{343} Opposite this pa.s.sage is written "_not botanically_," in Sir J. D. Hooker"s hand. The word _palms_ is underlined three times and followed by three exclamation marks. An explanatory note is added in the margin "singular paucity of palms and epiphytes in Trop.
Africa compared with Trop. America and Ind. Or." <=east indies="">.
Furthermore it may be observed that _all_ the organisms inhabiting any country are not perfectly adapted to it{344}; I mean by not being perfectly adapted, only that some few other organisms can generally be found better adapted to the country than some of the aborigines. We must admit this when we consider the enormous number of horses and cattle which have run wild during the three last centuries in the uninhabited parts of St Domingo, Cuba, and S. America; for these animals must have supplanted some aboriginal ones. I might also adduce the same fact in Australia, but perhaps it will be objected that 30 or 40 years has not been a sufficient period to test this power of struggling and overcoming the aborigines. We know the European mouse is driving before it that of New Zealand, like the Norway rat has driven before it the old English species in England. Scarcely an island can be named, where casually introduced plants have not supplanted some of the native species: in La Plata the Cardoon covers square leagues of country on which some S. American plants must once have grown: the commonest weed over the whole of India is an introduced Mexican poppy. The geologist who knows that slow changes are in progress, replacing land and water, will easily perceive that even if all the organisms of any country had originally been the best adapted to it, this could hardly continue so during succeeding ages without either extermination, or changes, first in the relative proportional numbers of the inhabitants of the country, and finally in their const.i.tutions and structure.{344} This partly corresponds to _Origin_, Ed. i. p. 337, vi. p.
483.