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adapted to all considerable diversities of heat, moisture, &c; but I fully admit that the mutual relations of organic beings are the most important, and as the number of species in any country goes on increasing, the organic conditions of life will become more and more complex^ Consequently, there seems at first to be no limit to the] amount of profitable diversification of structure, and there-1 fore no limit to the number of species which might be' produced. We do not know that even the most prolific area is fully stocked with specific forms; at the Cape of Good Hope, which supports such an astonishing number of species, many European plants have become naturalised. But geology shows us, at least within the whole immense Tertiary period, that the number of species of shells, and probably of mammals, has not increased. What, then, checks an indefinite increase in the number of species?; Firstly, the amount of life (I do not mean the number of | specific forms) supported on any area must have a limit, depending so largely as it does on physical conditions: therefore where very many species are supported, each, or nearly each, will be few in individuals; and any speciesl with scanty numbers would be liable to extermination/ from accidental fluctuations in the nature of seasons and! in the number of its enemies. The process of exterminaj tion would in such cases be rapid, whereas the process on the production of new species would always be slow. Im| agine the extreme case of as many species as individuals in England, and the first severe winter or very dry summer would exterminate thousands on thousands of species; and individuals of other species would take their places. > Secondly, I suspect that when any species becomes very I rare, close interbreeding will tend to exterminate it; at least authors have thought that this comes into play in accounting for the deterioration of aurochs in Lithuania, of red deer in Scotland, and of bears in Norway, &c Thirdly, as far as animals are concerned, some species are\ closely adapted to prey on some one other being; but if this other being had been rare, it would not have been any advantage to the animal to have been produced in close relation to its prey; therefore it would not have
been produced by natural selection. Fourthly, when any species becomes few in number, the process of modification will be slower, for the chance of favourable variations arising will be lessened; therefore if we suppose an area to be inhabited by very many species, each, or nearlyeach, species will be poor in individuals, and consequently the process of modification and of giving birth to new forms wil] be retarded. Fifthly, and this I am inclined to think is the most important element, a dominant species which has already beaten many competitors in its own home, will tend to spread and supplant many others. Alph. de Candolle has shown that those species which spread widely, tend generally to spread very widely; and consequently they will tend to exterminate several species in several areas, and thus check the inordinate increase of specific forms throughout the world. Hooker has recently shown that in the S. E. corner of Australia, where apparently there are many invaders from difi'erent quarters of the world, the endemic Australian species have apparently been greatly reduced in number. How much weight to attribute to these several causes, I do not pretend to assign; but conjointly I think they must limit in each country the tendency to an indefinite augmentation of specific forms.
Natural Selection acts, as we have seen, exclusively by the preservation and accumulation of variations, which are beneficial under the organic and inorganic conditions of life to which each creature is at each successive period exposed. The ultimate result will be that each creature will tend to become more and more improved in relation "to its conditions of life. This improvement will, I think, inevitably lead to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation. Amongst the Vertebrata, the degree of intellect and an approach in structure to man clearly come into play. It might be thought that the amount of change which the various parts and organs undergo in their development from the embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect and even monstrous, so that the mature animal cannot be called higher than its larva. Von Baer's standard seems the most widely applicable and the best; namely, the amount of differentiation of the different parts (in the adult state, as I should be inclined to add), and their specialisation for different functions; or as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure a subject this is, if we look, for instance, to Fish, amongst which some naturalists rank those as highest which, like the sharks, approach nearest to reptiles; whilst other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate orders. Still more plainly we see the obscurity of the subject, by turning to plants, where the standard of intellect is of course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals, petals, stamens and pistils, fully developed in each flower; whereas other botanists, probably with more truth, look at the plants which have their several organs much modified and somewhat reduced in number as being of the highest rank.
If we look at the differentiation and specialisation of the several organs of each being, when adult (and this will include the advancement of the brain for intellectual purposes), as the best standard of highness of organisa-!, tion, natural selection will clearly lead towards highness ;j for all physiologists admit that the specialisation of organs^ inasmuch as they perform in this state their functions bet-j ter, is an advantage to each being; and hence the accu-J mulation of variations tending towards specialisation is within the scope of natural selection. On the other hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every ill-occupied place in the economy of nature, that it is quite possible for natural selection gradually to fit an or
ganic being to a situation in which several organs would be superfluous and useless; and in such cases there might be retrogression in the scale of organisation. Whether organisation on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on geological succession.
But it may be objected, that if all organic beings thus tend to rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist, and how is it that in each great class some forms are far more highly-developed than others? Why have not the more highly-developed forms everywhere supplanted and exterminated the lower? Lamarck, who believed in an innate land inevitable tendency towards perfection in all organic [ beings, seems to have felt this difficulty so strongly, that »he was led to suppose that new and simple forms were continually being produced by spontaneous generation. I need hardly say that science in her progress has forbid den us to believe that living creatures are now ever produced from inorganic matter. On my theory the present existence of lowly organized productions offers no difficulty; for natural selection includes no necessary and universal law of advancement or development; it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life. And it may be asked, what advantage, as far as we can see, would it be to an infusorian animalcule—to an intestinal worm—or even to an earth-worm, to be highly organized? If it were no advantage, these forms would be left by natural selection unimproved or but little improved; and might remain for indefinite ages in their present little advanced condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state. But to suppose that most of the many now-existing low forms have not in the least advanced since the first dawn of life, would be rash; for every naturalist who has dissected some of the beings now ranked as very low in the scale, must often have been struck with their really wondrous and beautiful organisation.
Nearly the same remarks are applicable, if we look to the great existing differences in the grades of organisation -within almost every class, excepting birds; for instance, to the coexistence of mammals and lish in the vertebrata, —or to the coexistence of man and the ornithorhynchus amongst mammalia,—or amongst fish, of the shark and Amphioxus, which latter fish in the extreme simplicity of its structure closely approaches the invertebrate classes. But mammals and fish hardly come into competition with each other; the advancement of certain mammals or of the whole class to the highest grade of organisation would not lead to their taking the place of and thus exterminating fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aerial respiration ; so that warm-blooded mammals, when inhabiting the water, live under some disadvantages compared with fishes. In this latter class members of the shark family would not, it is probable, tend to supplant the Amphioxus; the struggle ibr existence in the case of the Amphioxus must lie with members of the invertebrate classes. The three lowest orders of mammals—namely, marsupials, edentata and rodents—coexist in South America in the same region with numerous monkeys. Although organisation, on the whole, may advance throughout the world, yet the scale of perfection will still present all degrees for the high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition. In some cases, as we shall hereafter see, lowly-organized forms seem to have been preserved to the present day, from having inhabited peculiar or isolated stations where they have been subjected to less severe competition; and where they have not advanced in organization owing to their scanty individual numbers, which, as already explained, retards the chance of favourable variations arising.
Finally, I believe that lowly-organised forms now exist in numbers throughout the world and in nearly every class, from various causes. In some cases favoura