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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 inore 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 organisation, natural selection will clearly lead towards highness ; for all physiologists admit that the specialisation of organs, inasmuch as they perform in this state their functions better, is an advantage to each being; and hence the accumulation 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 organic being to a situation in which several organs would be superfludus 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 and 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 fish 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 maminals 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 aërial 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 for 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 favourable variations may never have arisen for natural selection to act on and accumulate. In no case, perhaps, has time sufficed for the utmost possible maximum of development. In some few cases there may have been what we must call retrogression of organisation. But the main cause lies in the circumstance, that under very simple conditions of life, a high organisation would be of no servicepossibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and thus injured.
A difficulty, diametrically opposite to this which we have just been considering, might be advanced; namely, looking to the dawn of life, when all organic beings, as we may imagine, presented the simplest structure, how could the first steps in advancement or in the differentiation and specialization of parts arise? I can make no sufficient answer, and can only say that we have no facts to guide us, and therefore that all speculations on this subject would be baseless and useless.
Summary of Chapter.—If during the long course of ages and under varying conditions of life, organic Leings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection. Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily as the adult. Amongst many animals, sexual selection will give its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles with other males.
Whether natural selection has really thus acted in nature, in modifying and adapting the various forms of life to their several conditions and stations, must be judged of by the general tenour and balance of evidence given in the following chapters. But we already see how it entails extinction; and how largely extinction has acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking at the inhabitants of any small spot or at naturalised productions. Therefore during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified these descendants become, the better will be their chance of succeeding in the battle of life. Thus the small differences distinguishing varieties of the same species, will steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and widely-ranging species, belonging to the larger genera, which vary most; and these will tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. On these principles, I believe that the differences in rank of the innumerable organic beings in each class throughout the world, as well as the nature of their affinities, may be explained. It is a truly wonderful fact—the wonder of which we are apt to overlook from familiarity