« PreviousContinue »
RECAPITULATION 391 together all the species in each group by gradations as ,, fine as our present varieties, it may be asked, Why do@ we not see these linking forms all around us? Why are not all organic beings blended together in an inextricable chaos ‘I With respect to existing forms, we should remember that we have no right to expect (excepting in rare cases) to discover directly connecting links between them, but only between each and some extinct and supplanted form. Even on a wide area, which has during a long period remained continuous, and of which the climate and other conditions of life change insensibly in going from a district occupied by one species into another district occupied by a closely allied species, we have no just right to expect often to find intermediate varieties in the intermediate zone. For we have reason to believe that only a few species of a genus ever undergo change; the other species becoming utterly extinct and leaving no modified progeny. Of the species which do change, only a few within the same country change at the same time; and all modifications are slowly effected. I__hrave also shown that the intermediate varieties which wilf at'first probabfy exist in thji'intermediate zones, will be liable to be supplanted bllhe allied formson 'either'hand; and the latter, from existing in greater numbers, will generally be modified ,_1 ail imp‘rOved atra' quickerrrate than the intermediate f / varieties, which exist in lesser numbers; so that the intermediate varieties will, in the long run, he supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting links, between the living and extinct inhabitants of the world, and at each successive period between the extinct and still older species, why is not every geological formation charged with such links ‘2 Why does not every collection of fossil remains afford plain evidence of the gradation and mutation of the forms of life? Although geological research has un— doubtedly revealed the former existence of many links, bringing numerous forms of life much closer together, it does not yield the infinitely many fine gradations between past and present species required on my theory; and this is the most obvious and forcible of the many objections which may be urged against it. Why, again, do whole groups of allied species appear, though certainly they often falsely appear, to have come in suddenly on
the several geological stages? Why do we not find great piles of strata beneath the Silurian system, stored with the remains of the progenitors of the Silurian groups of fossils? For on my theory such strata must somewhere have been deposited at these ancient and utterly unknown epochs in the world’s history.
I can answer these questions and objections only on the supposition that the geological record is far more imperfect than most geologists believe. It cannot be objected that there has not been time sufficient for any amount of organic change ; for the lapse of time has been so great as to‘be utterly inappreciable by the human intellect. The number of specimens in all our museums is absolutely as nothing compared with the countless generations of countless species which certainly have existed. The parent form of any two or more species would not be in all its characters directly intermediate between its modified offspring, any more than the rockpigeon is directly intermediate in crop and tail between its descendants the pouter and fantail pigeons. We should not be able to recognise a species as the parent of_ another species if we were to examine both ever so closely, unless we likewise possessed many of the intermediate links between their past and present states; and these many links we could hardly ever expect to discover, owing to the imperfection of the geological record. If two or three, or even more linking forms were discovered, they would simply be ranked as so many new species, more especially if found in different geological sub-stages, let their differences be ever so slight. Numerous existing doubtful forms could be named which are probably varieties ; but who will pretend that in future ages so many fossil links will. be discovered, that naturalists will be able to decide, on the common view, whether or not these doubtful forms are varieties ‘I 0nly a small portion of the world has been geologically explored. Only organic beings of certain classes can be preserved in a fossil condition, at least in any great number. Widely ranging species vary most, and varieties are often at first local—both causes rendering the discovery of intermediate links less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they do spread, it discovered in a geological formation, they will appear as if suddenly created there, and will be simply classed as new species. Most formations have been intermittent in their accumulation; and their duration, . I am inclined to believe, has been shorter than the average duration of specific forms. Successive formations are in most cases separated from each other by enormous blank intervals of time; for fossiliferous formations thick enough to resist future degradation can generally be accumulated only where much sediment is deposited on the subsiding bed of the sea. During the alternate periods of elevation and of stationary level the record will be generally blank. During theselatter periods there will probably be more variability in the forms of life; during periods of subsié dence, more extinction.
With respect to the absence of fossiliferous formations beneath the lowest Silurian strata, I can only recur to the hypothesis given in. the ninth chapter. That the geological record is imperfect all will admit; but that it is imperfect to the degree which I require, few will be inclined to admit. If we look to long enough intervals of time, geology plainly declares that all species have changed; and they have changed in the manner which my theory requires, for they have changed slowly and in a graduated manner. We clearly see this in the fossil remains from consecutive formations invariably being much more closely related to each other, than are the fossils from formations distant from each other in time.
Such is the sum of the several chief objections and difficulties which may justly be urged against my theory ; and I have now briefly recapitulated the answers and explanations which can be, given to them. I have felt these difficulties far too heavily during many years to doubt their weight. But it deserves especial notice that the more important objections relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are. We do not know all the possible transitional gradations between the simplest and the most perfect organs; it cannot be pretended that we know all the varied means of DistributiOn during the long lapse of years, or that we know how imperfect the Geological Record is. Grave as, thesev several difficulties are, in my judgment they do not overthrow the theory of descent from a few primordial forms with, subsequent modification.
Now let us turn to the other side of the argument, Under domestication we see much variability. This seems to be mainly due to the reproductive system being eminently susceptible to changes in the conditions of life ; so that this system, when not rendered impotent, fails to reproduce offspring exactly likely the parent-form. Variability is governed by many complex laws,—by correlation of growth, by use and disuse, and by the direct action of the physical conditions of life. There is much difficulty in ascertaining how much modification our domestic productions have undergone; but we may safely infer that the amount has been large, and that modifications can be inherited for long periods. As long as the conditions of life remain the same, we have reason to believe that a modification, which has already been inherited for many generations, may continue to be “inherited for an almost infinite number of generations. On the other hand we have evidence that variability, when it has once come into play, does not wholly cease; for new varieties are still occasionally produced by our most anciently domesticated productions.
Man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability. But man can and does select the variations given to him by nature, and thus accumulate them in any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time without any thought of altering the breed. It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be inappreciable by an uneducated eye. This process of selection has been the great agency in the production of the most distinct and useful domestic breeds. That many of the breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether very many of them are varieties or aboriginally distinct species. .
There is no obvious reason why the principles which have acted so efficiently under domestication should not have acted under nature; In the preservation of favoured individuals and races, during the constantly-recurrent Struggle for Existence, we see the most powerful and ever-actingmeans of selection. The struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings. This high rate of increase is proved by calculation,-—by the rapid increase of many animals and plants during a succession of peculiar s‘easons, or when naturalised in a new country. More individuals are born than can possibly survive. >A grain in the balance will determine which individual shall live and which shall die,—which variety or species shall increase in number, and which shall decrease, or finally become extinct. As the individuals of the same species come in all respects into the closest competition with each other, the struggle will generally be most severe between them ; it will be almost equally severe between the varieties of the same species, and next in severity between the species of the same genus. But the struggle will often be very severe between beings most remote in the scale of nature. The slightest advantage in one being, at any age or during any season, over those with which it comes into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will turn the balance.
With animals having separated sexes there Will in most cases be a struggle between the males for possession of the females. The most vigorous individuals, or those which have most successfully struggled with their conditions of life, will generally leave most progeny. But success will often depend on having special weapons or means of defence, or_on the charms of the males; and the slightest advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical changes, we might have expected that organic beings would have varied under nature, in the same‘way as they generally have varied under the changed conditions of domestication. And if there be any variability under nature, it would be an unaccountable fact if natural selection had not come into play. It has often been asserted, but the assertion is quite incapable of proof, that the amount of variation under nature is a strictly limited quantity. Man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productiOns; and every one admits that there are at least individual differ