{420} "Nature may almost be said to have guarded against the frequent discovery of her transitional or linking forms," _Origin_, Ed. i. p. 292. A similar but not identical pa.s.sage occurs in _Origin_, Ed. vi. p. 428.
{421} _Origin_, Ed. i. p. 291, vi. p. 426.
{422} _Origin_, Ed. i. p. 288, vi. p. 422.
{423} _Origin_, Ed. i. p. 289, vi. p. 423.
Hence many more remains will be preserved to a distant age, in any region of the world, during periods of its subsidence{424}, than of its elevation.
{424} _Origin_, Ed. i. p. 300, vi. p. 439.
But during the subsidence of a tract of land, its inhabitants (as before shown) will from the decrease of s.p.a.ce and of the diversity of its stations, and from the land being fully preoccupied by species fitted to diversified means of subsistence, be little liable to modification from selection, although many may, or rather must, become extinct. With respect to its circ.u.m-marine inhabitants, although during a change from a continent to a _great_ archipelago, the number of stations fitted for marine beings will be increased, their means of diffusion (an important check to change of form) will be greatly improved; for a continent stretching north and south, or a quite open s.p.a.ce of ocean, seems to be to them the only barrier. On the other hand, during the elevation of a small archipelago and its conversion into a continent, we have, whilst the number of stations are increasing, both for aquatic and terrestrial productions, and whilst these stations are not fully preoccupied by perfectly adapted species, the most favourable conditions for the selection of new specific forms; but few of them in their early transitional states will be preserved to a distant epoch. We must wait during an enormous lapse of time, until long-continued subsidence shall have taken the place in this quarter of the world of the elevatory process, for the best conditions of the embedment and the preservation of its inhabitants. Generally the great ma.s.s of the strata in every country, from having been chiefly acc.u.mulated during subsidence, will be the tomb, not of transitional forms, but of those either becoming extinct or remaining unmodified.
The state of our knowledge, and the slowness of the changes of level, do not permit us to test the truth of these remarks, by observing whether there are more transitional or "fine" (as naturalists would term them) species, on a rising and enlarging tract of land, than on an area of subsidence. Nor do I know whether there are more "fine" species on isolated volcanic islands in process of formation, than on a continent; but I may remark, that at the Galapagos Archipelago the number of forms, which according to some naturalists are true species, and according to others are mere races, is considerable: this particularly applies to the different species or races of the same genera inhabiting the different islands of this archipelago. Furthermore it may be added (as bearing on the great facts discussed in this chapter) that when naturalists confine their attention to any one country, they have comparatively little difficulty in determining what forms to call species and what to call varieties; that is, those which can or cannot be traced or shown to be probably descendants of some other form: but the difficulty increases, as species are brought from many stations, countries and islands. It was this increasing (but I believe in few cases insuperable) difficulty which seems chiefly to have urged Lamarck to the conclusion that species are mutable.
CHAPTER VII
ON THE NATURE OF THE AFFINITIES AND CLa.s.sIFICATION OF ORGANIC BEINGS{425}
{425} Ch. XIII of the _Origin_, Ed. i., Ch. XIV Ed. vi. begins with a similar statement. In the present Essay the author adds a note:--"The obviousness of the fact (_i.e._ the natural grouping of organisms) alone prevents it being remarkable. It is scarcely explicable by creationist: groups of aquatic, of vegetable feeders and carnivorous, &c., might resemble each other; but why as it is.
So with plants,--a.n.a.logical resemblance thus accounted for. Must not here enter into details." This argument is incorporated with the text in the _Origin_, Ed. i.
_Gradual appearance and disappearance of groups._
It has been observed from the earliest times that organic beings fall into groups{426}, and these groups into others of several values, such as species into genera, and then into sub-families, into families, orders, &c. The same fact holds with those beings which no longer exist.
Groups of species seem to follow the same laws in their appearance and extinction{427}, as do the individuals of any one species: we have reason to believe that, first, a few species appear, that their numbers increase; and that, when tending to extinction, the numbers of the species decrease, till finally the group becomes extinct, in the same way as a species becomes extinct, by the individuals becoming rarer and rarer. Moreover, groups, like the individuals of a species, appear to become extinct at different times in different countries. The Palaeotherium was extinct much sooner in Europe than in India: the Trigonia{428} was extinct in early ages in Europe, but now lives in the seas of Australia. As it happens that one species of a family will endure for a much longer period than another species, so we find that some whole groups, such as Mollusca, tend to retain their forms, or to remain persistent, for longer periods than other groups, for instance than the Mammalia. Groups therefore, in their appearance, extinction, and rate of change or succession, seem to follow nearly the same laws with the individuals of a species{429}.
{426} _Origin_, Ed. i. p. 411, vi. p. 566.
{427} _Origin_, Ed. i. p. 316, vi. p. 457.
{428} _Origin_, Ed. i. p. 321, vi. p. 463.
{429} In the _Origin_, Ed. i. this preliminary matter is replaced (pp. 411, 412, vi. pp. 566, 567) by a discussion in which extinction is also treated, but chiefly from the point of view of the theory of divergence.
_What is the Natural System?_
The proper arrangement of species into groups, according to the natural system, is the object of all naturalists; but scarcely two naturalists will give the same answer to the question, What is the natural system and how are we to recognise it? The most important characters{430} it might be thought (as it was by the earliest cla.s.sifiers) ought to be drawn from those parts of the structure which determine its habits and place in the economy of nature, which we may call the final end of its existence. But nothing is further from the truth than this; how much external resemblance there is between the little otter (Chironectes) of Guiana and the common otter; or again between the common swallow and the swift; and who can doubt that the means and ends of their existence are closely similar, yet how grossly wrong would be the cla.s.sification, which put close to each other a Marsupial and Placental animal, and two birds with widely different skeletons. Relations, such as in the two latter cases, or as that between the whale and fishes, are denominated "a.n.a.logical{431}," or are sometimes described as "relations of adaption." They are infinitely numerous and often very singular; but are of no use in the cla.s.sification of the higher groups. How it comes, that certain parts of the structure, by which the habits and functions of the species are settled, are of no use in cla.s.sification, whilst other parts, formed at the same time, are of the greatest, it would be difficult to say, on the theory of separate creations.
{430} _Origin_, Ed. i. p. 414, vi. p. 570.
{431} _Origin_, Ed. i. p. 414, vi. p. 570.
Some authors as Lamarck, Whewell &c., believe that the degree of affinity on the natural system depends on the degrees of resemblance in organs more or less physiologically important for the preservation of life. This scale of importance in the organs is admitted to be of difficult discovery. But quite independent of this, the proposition, as a general rule, must be rejected as false; though it may be partially true. For it is universally admitted that the same part or organ, which is of the highest service in cla.s.sification in one group, is of very little use in another group, though in both groups, as far as we can see, the part or organ is of equal physiological importance: moreover, characters quite unimportant physiologically, such as whether the covering of the body consists of hair or feathers, whether the nostrils communicated with the mouth{432} &c., &c., are of the highest generality in cla.s.sification; even colour, which is so inconstant in many species, will sometimes well characterise even a whole group of species. Lastly, the fact, that no one character is of so much importance in determining to what great group an organism belongs, as the forms through which the embryo{433} pa.s.ses from the germ upwards to maturity, cannot be reconciled with the idea that natural cla.s.sification follows according to the degrees of resemblance in the parts of most physiological importance. The affinity of the common rock-barnacle with the Crustaceans can hardly be perceived in more than a single character in its mature state, but whilst young, locomotive, and furnished with eyes, its affinity cannot be mistaken{434}. The cause of the greater value of characters, drawn from the early stages of life, can, as we shall in a succeeding chapter see, be in a considerable degree explained, on the theory of descent, although inexplicable on the views of the creationist.
{432} These instances occur with others in the _Origin_, Ed. i. p.
416, vi. p. 572.
{433} _Origin_, Ed. i. p. 418, vi. p. 574.
{434} _Origin_, Ed. i. pp. 419, 440, vi. pp. 575, 606.
Practically, naturalists seem to cla.s.sify according to the resemblance of those parts or organs which in related groups are most uniform, or vary least{435}: thus the aestivation, or manner in which the petals etc.
are folded over each other, is found to afford an unvarying character in most families of plants, and accordingly any difference in this respect would be sufficient to cause the rejection of a species from many families; but in the Rubiaceae the aestivation is a varying character, and a botanist would not lay much stress on it, in deciding whether or not to cla.s.s a new species in this family. But this rule is obviously so arbitrary a formula, that most naturalists seem to be convinced that something ulterior is represented by the natural system; they appear to think that we only discover by such similarities what the arrangement of the system is, not that such similarities make the system. We can only thus understand Linnaeus"{436} well-known saying, that the characters do not make the genus; but that the genus gives the characters: for a cla.s.sification, independent of characters, is here presupposed. Hence many naturalists have said that the natural system reveals the plan of the Creator: but without it be specified whether order in time or place, or what else is meant by the plan of the Creator, such expressions appear to me to leave the question exactly where it was.
{435} _Origin_, Ed. i. pp. 418, 425, vi. pp. 574, 581.
{436} _Origin_, Ed. i. p. 413, vi. p. 569.
Some naturalists consider that the geographical position{437} of a species may enter into the consideration of the group into which it should be placed; and most naturalists (either tacitly or openly) give value to the different groups, not solely by their relative differences in structure, but by the number of forms included in them. Thus a genus containing a few species might be, and has often been, raised into a family on the discovery of several other species. Many natural families are retained, although most closely related to other families, from including a great number of closely similar species. The more logical naturalist would perhaps, if he could, reject these two contingents in cla.s.sification. From these circ.u.mstances, and especially from the undefined objects and criterions of the natural system, the number of divisions, such as genera, sub-families, families, &c., &c., has been quite arbitrary{438}; without the clearest definition, how can it be possible to decide whether two groups of species are of equal value, and of what value? whether they should both be called genera or families; or whether one should be a genus, and the other a family{439}?
{437} _Origin_, Ed. i. pp. 419, 427, vi. pp. 575, 582.
{438} This is discussed from the point of view of divergence in the _Origin_, Ed. i. pp. 420, 421, vi. pp. 576, 577.
{439} I discuss this because if Quinarism true, I false. _On the kind of relation between distinct groups._
I have only one other remark on the affinities of organic beings; that is, when two quite distinct groups approach each other, the approach is _generally_ generic{440} and not special; I can explain this most easily by an example: of all Rodents the Bizcacha, by certain peculiarities in its reproductive system, approaches nearest to the Marsupials; of all Marsupials the Phascolomys, on the other hand, appears to approach in the form of its teeth and intestines nearest to the Rodents; but there is no special relation between these two genera{441}; the Bizcacha is no nearer related to the Phascolomys than to any other Marsupial in the points in which it approaches this division; nor again is the Phascolomys, in the points of structure in which it approaches the Rodents, any nearer related to the Bizcacha than to any other Rodent.
Other examples might have been chosen, but I have given (from Waterhouse) this example as it ill.u.s.trates another point, namely, the difficulty of determining what are a.n.a.logical or adaptive and what real affinities; it seems that the teeth of the Phascolomys though _appearing closely_ to resemble those of a Rodent are found to be built on the Marsupial type; and it is thought that these teeth and consequently the intestines may have been adapted to the peculiar life of this animal and therefore may not show any real relation. The structure in the Bizcacha that connects it with the Marsupials does not seem a peculiarity related to its manner of life, and I imagine that no one would doubt that this shows a real affinity, though not more with any one Marsupial species than with another. The difficulty of determining what relations are real and what a.n.a.logical is far from surprising when no one pretends to define the meaning of the term relation or the ulterior object of all cla.s.sification. We shall immediately see on the theory of descent how it comes that there should be "real" and "a.n.a.logical" affinities; and why the former alone should be of value in cla.s.sification--difficulties which it would be I believe impossible to explain on the ordinary theory of separate creations.
{440} In the corresponding pa.s.sage in the _Origin_, Ed. i. p. 430, vi. p. 591, the term _general_ is used in place of _generic_, and seems a better expression. In the margin the author gives Waterhouse as his authority.
{441} _Origin_, Ed. i. p. 430, vi. p. 591.
_Cla.s.sification of Races or Varieties._
Let us now for a few moments turn to the cla.s.sification of the generally acknowledged varieties and subdivisions of our domestic beings{442}; we shall find them systematically arranged in groups of higher and higher value. De Candolle has treated the varieties of the cabbage exactly as he would have done a natural family with various divisions and subdivisions. In dogs again we have one main division which may be called the _family_ of hounds; of these, there are several (we will call them) _genera_, such as blood-hounds, fox-hounds, and harriers; and of each of these we have different _species_, as the blood-hound of Cuba and that of England; and of the latter again we have breeds truly producing their own kind, which may be called races or varieties. Here we see a cla.s.sification practically used which typifies on a lesser scale that which holds good in nature. But amongst true species in the natural system and amongst domestic races the number of divisions or groups, inst.i.tuted between those most alike and those most unlike, seems to be quite arbitrary. The number of the forms in both cases seems practically, whether or not it ought theoretically, to influence the denomination of groups including them. In both, geographical distribution has sometimes been used as an aid to cla.s.sification{443}; amongst varieties, I may instance, the cattle of India or the sheep of Siberia, which from possessing some characters in common permit a cla.s.sification of Indian and European cattle, or Siberian and European sheep. Amongst domestic varieties we have even something very like the relations of "a.n.a.logy" or "adaptation{444}"; thus the common and Swedish turnip are both artificial varieties which strikingly resemble each other, and they fill nearly the same end in the economy of the farm-yard; but although the swede so much more resembles a turnip than its presumed parent the field cabbage, no one thinks of putting it out of the cabbages into the turnips. Thus the greyhound and racehorse, having been selected and trained for extreme fleetness for short distances, present an a.n.a.logical resemblance of the same kind, but less striking as that between the little otter (Marsupial) of Guiana and the common otter; though these two otters are really less related than
{442} In a corresponding pa.s.sage in the _Origin_, Ed. i. p. 423, vi. p. 579, the author makes use of his knowledge of pigeons. The pseudo-genera among dogs are discussed in _Var. under Dom._, Ed.
ii. vol. I. p. 38.
{443} _Origin_, Ed. i. pp. 419, 427, vi. pp. 575, 582.
{444} _Origin_, Ed. i. pp. 423, 427, vi. pp. 579, 583.
{445} _Origin_, Ed. i. p. 423, vi. p. 579.
I think it cannot be doubted that the main cause of all the varieties which have descended from the aboriginal dog or dogs, or from the aboriginal wild cabbage, not being equally like or unlike--but on the contrary, obviously falling into groups and sub-groups--must in chief part be attributed to different degrees of true relationship; for instance, that the different kinds of blood-hound have descended from one stock, whilst the harriers have descended from another stock, and that both these have descended from a different stock from that which has been the parent of the several kinds of greyhound. We often hear of a florist having some choice variety and breeding from it a whole group of sub-varieties more or less characterised by the peculiarities of the parent. The case of the peach and nectarine, each with their many varieties, might have been introduced. No doubt the relationship of our different domestic breeds has been obscured in an extreme degree by their crossing; and likewise from the slight difference between many breeds it has probably often happened that a "sport" from one breed has less closely resembled its parent breed than some other breed, and has therefore been cla.s.sed with the latter. Moreover the effects of a similar climate{446} may in some cases have more than counterbalanced the similarity, consequent on a common descent, though I should think the similarity of the breeds of cattle of India or sheep of Siberia was far more probably due to the community of their descent than to the effects of climate on animals descended from different stocks.