I have discussed the probable origin of domestic pigeons at some, yet quiteinsufficient, length; because when I first kept pigeons and watched the severalkinds, knowing well how true they bred, I felt fully as much difficulty inbelieving that they could ever have descended from a common parent, as anynaturalist could in coming to a similar conclusion in regard to the manyspecies of finches, or other large groups of birds, in nature. With these exceptions (and with that of the perfect fertilityof varieties when crossed,—a subject hereafter to be discussed), domesticraces of the same species differ from each other in the same manner as, only inmost cases in a lesser degree than, do closely-allied species of the same genusin a state of nature. If it could be shown that our domestic varieties manifested astrong tendency to reversion,—that is, to lose their acquired characters,whilst kept under unchanged conditions, and whilst kept in a considerable body,so that free intercrossing might check, by blending together, any slightdeviations of structure, in such case, I grant that we could deduce nothingfrom domestic varieties in regard to species. Nevertheless, as ourvarieties certainly do occasionally revert in some of their characters toancestral forms, it seems to me not improbable, that if we could succeed innaturalising, or were to cultivate, during many generations, the several races,for instance, of the cabbage, in very poor soil (in which case, however, someeffect would have to be attributed to the direct action of the poor soil), thatthey would to a large extent, or even wholly, revert to the wild aboriginalstock.
Ihave, therefore, discussed this case, at some little but wholly insufficientlength, in order to show the power of natural selection, and likewise becausethis is by far the most serious special difficulty, which my theory hasencountered. But I am bound toconfess, that, with all my faith in this principle, I should never haveanticipated that natural selection could have been efficient in so high adegree, had not the case of these neuter insects convinced me of the fact. Andnature has, as I believe, effected this admirable division of labour in thecommunities of ants, by the means of natural selection. It will indeed be thought that I have an overweening confidence in theprinciple of natural selection, when I do not admit that such wonderful andwell-established facts at once annihilate my theory.
Challenges to natural selection
If we look to the islands off the American shore, however much theymay differ in geological structure, the inhabitants, though they may be allpeculiar species, are essentially American. On these sameplains of La Plata, we see the agouti and bizcacha, animals having nearly thesame habits as our hares and rabbits and belonging to the same order ofRodents, but they plainly display an American type of structure. A third great fact, partly included in the foregoing statements, is theaffinity of the productions of the same continent or sea, though the speciesthemselves are distinct at different points and stations.
Nature and structure of Darwin’s argument
- The same varieties of the cabbage donot yield abundant and nutritious foliage and a copious supply of oil-bearingseeds.
- On the other hand, I can see no reason to doubt thatnatural selection will continually tend to preserve those individuals which areborn with constitutions best adapted to their native countries.
- Thevarieties, or incipient species, thus produced ultimately become converted, asI believe, into new and distinct species; and these, on the principle ofinheritance, tend to produce other new and dominantspecies.
- In many other instances, as in the several districts of the samecontinent, pre-occupation has probably played an important part in checking thecommingling of species under the same conditions of life.
- No two marine faunas are moredistinct, with hardly a fish, shell, or crab in common, than those of theeastern and western shores of South and Central America; yet these great faunasare separated only by the narrow, but impassable, isthmus of Panama.
For several hen ostriches, at least in the case ofthe American species, unite and lay first a few eggs in one nest and then inanother; and these are hatched by the males. But the Americancuckoo is in this predicament; for she makes her own nest and has eggs andyoung successively hatched, all at the same time. If thiswere the case, the process of laying and hatching might be inconveniently long,more especially as she has to migrate at a very early period; and the firsthatched young would probably have to be fed by the male alone. Insome cases compulsory habit alone has sufficed to produce such inherited mentalchanges; in other cases compulsory habit has done nothing, and all has been theresult of selection, pursued both methodically and unconsciously; but in mostcases, probably, habit and selection have acted together. Familiarityalone prevents our seeing how universally and largely the minds of our domesticanimals have been modified by domestication.
Asit is difficult to imagine that eyes, though useless, could be in any wayinjurious to animals living in darkness, I attribute their loss wholly todisuse. It is well known that several animals, belonging to the most different classes,which inhabit the caves of Styria and of Kentucky, are blind. For during thousandsof successive generations each individual beetle which flew least, either fromits wings having been ever so little less perfectly developed or from indolenthabit, will have had the best chance of surviving from not being blown out tosea; and, on the other hand, those beetles which most readily took to flightwill oftenest have been blown to sea and thus have been destroyed. As Professor Owen has remarked, there is nogreater anomaly in nature than a bird that cannot fly; yet there are several inthis state. Effects of Use and Disuse.—From the facts alluded to in the firstchapter, I think there can be little doubt that use in our domestic animalsstrengthens and enlarges certain parts, and disuse diminishes them; and thatsuch modifications are inherited.
This is the most interestingdepartment of natural history, and may be said to be its very soul. This resemblance is often expressed by the term“unity of type;” or by saying that the several parts and organs inthe different species of the class are homologous. We can clearly see how it isthat all living and extinct forms can be grouped together in one great system;and how the several members of each class are connected together by the mostcomplex and radiatinglines of affinities. Wecan understand why we value certain resemblances far more than others; why weare permitted to use rudimentary and useless organs, or others of triflingphysiological importance; why, in comparing one group with a distinct group, wesummarily reject analogical or adaptive characters, and yet use these samecharacters within the limits of the same group.
Variation under domestication and under nature
It is a fact of some little importance to us, that peculiaritiesappearing in the males of our domestic breeds are often transmitted eitherexclusively, or in a much greater degree, to males alone. Perhaps the correct way of viewing the wholesubject, would be, to look at the inheritance of every character whatever asthe rule, and non-inheritance as the anomaly. But the number anddiversity of inheritable deviations of structure, both those of slight andthose of considerable physiological importance, is endless. The whole organisation seems tohave become plastic, and tends to depart in some small degree from that of theparental type. Hence, ifman goes on selecting, and thus augmenting, any peculiarity, he will almostcertainly unconsciously modify other parts of the structure, owing to themysterious laws of the correlation of growth.
Summary of Darwin’s theory
Hence we may safelyinfer that icebergs formerly landed their rocky burthens on the shores of thesemid-ocean islands, and it is at least possible that they may have broughtthither the seeds of northern plants. Reflect for a moment on the millions of quails whichannually cross the Mediterranean; and can we doubt that the earth adhering totheir feet would sometimes include a few minute seeds? Mr. Brent informs me that a friend of his had togive up flying carrier-pigeons from France to England, as the hawks on theEnglish coast destroyed so many on their arrival. The fact of the larger fruits often floatinglonger than the small, is interesting; as plants with large seeds or fruitcould hardly be transported by any other means; and Alph.
LAWS OF VARIATION.
- But a far moreimportant consideration, as I believe, is that, during the process of furthermodification, by which two varieties are supposed on my theory to be convertedand perfected into two distinct species, the two which exist in larger numbersfrom inhabiting larger areas, will have a great advantage over the intermediatevariety, which existsin smaller numbers in a narrow and intermediate zone.
- The case of the Jerusalem artichoke, whichis never propagated by seed, and of which consequently new varieties have notbeen produced, has even been advanced—for it is now as tender as ever itwas—as proving that acclimatisation cannot be effected!
- Many years ago, when comparing,and seeing others compare, the birds from the separate islands of the GalapagosArchipelago, both one with another, and with those from the American mainland,I was much struck how entirely vague and arbitrary is the distinction betweenspecies and varieties.
- What a strange anomaly it would be,if, when coming one step lower in the series, to the individuals of the samespecies, a directly opposite rule prevailed; and species were not local, buthad been produced in two or more distinct areas!
- So, again, I was told that the foals of cart and race-horsesdiffered as much as the full-grown animals; and this surprised me greatly, as Ithink it probable that the difference between these two breeds has been whollycaused by selection under domestication; but having had careful measurementsmade of the dam and of a three-days old colt of a race and heavy cart-horse, Ifind that the colts have by no means acquired their full amount of proportionaldifference.
A long list could easily be given of “sporting plants;” by thisterm gardeners mean a single bud or offset, which suddenly assumes a new andsometimes very different character from that of the rest of the plant.Such buds can be propagated by grafting, etc., and sometimes by seed. This is generally attributed to vitiated instincts; but how manycultivated plants display the utmost vigour, and yet rarely or never seed! Geoffroy St.Hilaire’s experiments show that unnatural treatment of the embryo causesmonstrosities; and monstrosities cannot be separated by any clear line ofdistinction from mere variations. No one ought to feel surprise at much remaining as yet unexplained in regard tothe origin of species and varieties, if he makes due allowance for our profoundignorance in regard to the mutual relations of all the beings which live aroundus. This fundamental subject of Natural Selection will be treated at some length inthe fourth chapter; and we shall then see how Natural Selection almostinevitably causes much Extinction of the less improved forms of life andinduces what I have called Divergence of Character.
It is certain, on the one hand, that the sterility of various species whencrossed is so chicken game download different in degree and graduates away so insensibly, and, on theother hand, that the fertility of pure species is so easily affected by variouscircumstances, that for all practical purposes it is most difficult to saywhere perfect fertility ends and sterility begins. Thatthese processes are often injurious to the fertility of a plant cannot bedoubted; for Gärtner gives in his table about a score of cases of plants whichhe castrated, and artificially fertilised with their own pollen, and (excludingall cases such as the Leguminosæ, in which there is an acknowledged difficultyin the manipulation) half of these twenty plants had their fertility in somedegree impaired. He alwayscompares the maximum number of seeds produced by two species when crossed andby their hybrid offspring, with the average number produced by both pureparent-species in a state of nature. This distinction is important,when the cause of the sterility, which is common to the two cases, has to beconsidered.
Can the principle of selection, which we have seenis so potent in the hands of man, apply in nature? When we reflect on this struggle, we may console ourselveswith the full belief, that the war of nature is not incessant, that no fear isfelt, that death is generally prompt, and that the vigorous, the healthy, andthe happy survive and multiply. Not until we reach the extreme confines of life, in thearctic regions or on the borders of an utter desert, will competition cease.The land may be extremely cold or dry, yet there will be competition betweensome few species, or between the individuals of the same species, for thewarmest or dampest spots.
Struggle for existence, natural selection, and divergence
Having alluded to the subject of reversion, I may here refer to a statementoften made by naturalists—namely, that our domestic varieties, when runwild, gradually but certainly revert in character to their aboriginal stocks.Hence it has been argued that no deductions can be drawn from domestic races tospecies in a state of nature. While some commentators have taken this as a concession to religion that Darwin later regretted, Darwin’s view at the time was of God creating life through the laws of nature, and even in the first edition there are several references to “creation”. The minority view of August Weismann, that natural selection was the only mechanism, was called neo-Darwinism. Despite this, Huxley strongly supported Darwin on evolution; though he called for experiments to show whether natural selection could form new species, and questioned if Darwin’s gradualism was sufficient without sudden leaps to cause speciation. The naturalism of natural selection conflicted with presumptions of purpose in nature and while this could be reconciled by theistic evolution, other mechanisms implying more progress or purpose were more acceptable. I think all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms.
Thus we can account forthe fact that all organisms, recent and extinct, are included under a few greatorders, under still fewer classes, and all in one great natural system. On my view of characters being of real importance for classification,only in so far as they reveal descent, we can clearly understand why analogicalor adaptive character, although of the utmost importance to the welfare of thebeing, are almost valueless to the systematist. Lamarck first calledattention to this distinction, and he has been ably followed by Macleay andothers. We can understand why a species or a group of species may depart, in several ofits most important characteristics, from its allies, and yet be safely classedwith them. He who believes that the cowslip isdescended from the primrose, or conversely, ranks them together as a singlespecies, and gives a single definition.
Thus, on the viewwhich I hold, the natural system is genealogical in its arrangement, like apedigree; but the degrees of modification which the different groups haveundergone, have to be expressed by ranking them under different so-calledgenera, sub-families, families, sections, orders, and classes. Instances could be given amongst plants and insects, of agroup of forms, first ranked by practised naturalists as only a genus, and thenraised to the rank of a sub-family or family; and this has been done, notbecause further research has detected important structural differences, atfirst overlooked, but because numerous allied species, with slightly differentgrades of difference, have been subsequently discovered. But their importancefor classification, I believe, depends on their greater constancy throughoutlarge groups of species; and this constancy depends on such organs havinggenerally been subjected to less change in the adaptation of the species totheir conditions of life. These five genera have also much, though less, in common; andthey form a family distinct from that including the three genera still furtherto the right hand, which diverged at a still earlier period. Groups of species, belonging either to a certain period oftime, or to a certain area, are often characterised by trifling characters incommon, as of sculpture or colour.
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