物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 3. STRUGGLE FOR EXISTENCE. Page 2
Many cases are on record showing how complex and unexpected are thechecks and relations between organic beings, which have to struggletogether in the same country. I will give only a single instance,which, though a simple one, has interested me. In Staffordshire, onthe estate of a relation where I had ample means of investigation,there was a large and extremely barren heath, which had never beentouched by the hand of man; but several hundred acres of exactly thesame nature had been enclosed twenty-five years previously and plantedwith Scotch fir. The change in the native vegetation of the plantedpart of the heath was most remarkable, more than is generally seen inpassing from one quite different soil to another: not only theproportional numbers of the heath-plants were wholly changed, buttwelve species of plants (not counting grasses and carices) flourishedin the plantations, which could not be found on the heath. The effecton the insects must have been still greater, for six insectivorousbirds were very common in the plantations, which were not to be seenon the heath; and the heath was frequented by two or three distinctinsectivorous birds. Here we see how potent has been the effect of theintroduction of a single tree, nothing whatever else having been done,with the exception that the land had been enclosed, so that cattlecould not enter. But how important an element enclosure is, I plainlysaw near Farnham, in Surrey. Here there are extensive heaths, with afew clumps of old Scotch firs on the distant hill-tops: within thelast ten years large spaces have been enclosed, and self-sown firs arenow springing up in multitudes, so close together that all cannotlive.
When I ascertained that these young trees had not been sown orplanted, I was so much surprised at their numbers that I went toseveral points of view, whence I could examine hundreds of acres ofthe unenclosed heath, and literally I could not see a single Scotchfir, except the old planted clumps. But on looking closely between thestems of the heath, I found a multitude of seedlings and little trees,which had been perpetually browsed down by the cattle. In one squareyard, at a point some hundred yards distant from one of the oldclumps, I counted thirty-two little trees; and one of them, judgingfrom the rings of growth, had during twenty-six years tried to raiseits head above the stems of the heath, and had failed. No wonder that,as soon as the land was enclosed, it became thickly clothed withvigorously growing young firs. Yet the heath was so extremely barrenand so extensive that no one would ever have imagined that cattlewould have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of theScotch fir; but in several parts of the world insects determine theexistence of cattle. Perhaps Paraguay offers the most curious instanceof this; for here neither cattle nor horses nor dogs have ever runwild, though they swarm southward and northward in a feral state; andAzara and Rengger have shown that this is caused by the greater numberin Paraguay of a certain fly, which lays its eggs in the navels ofthese animals when first born. The increase of these flies, numerousas they are, must be habitually checked by some means, probably bybirds. Hence, if certain insectivorous birds (whose numbers areprobably regulated by hawks or beasts of prey) were to increase inParaguay, the flies would decrease--then cattle and horses wouldbecome feral, and this would certainly greatly alter (as indeed I haveobserved in parts of South America) the vegetation: this again wouldlargely affect the insects; and this, as we just have seen inStaffordshire, the insectivorous birds, and so onwards inever-increasing circles of complexity. We began this series byinsectivorous birds, and we have ended with them. Not that in naturethe relations can ever be as simple as this. Battle within battle mustever be recurring with varying success; and yet in the long-run theforces are so nicely balanced, that the face of nature remains uniformfor long periods of time, though assuredly the merest trifle wouldoften give the victory to one organic being over another. Neverthelessso profound is our ignorance, and so high our presumption, that wemarvel when we hear of the extinction of an organic being; and as wedo not see the cause, we invoke cataclysms to desolate the world, orinvent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,most remote in the scale of nature, are bound together by a web ofcomplex relations. I shall hereafter have occasion to show that theexotic Lobelia fulgens, in this part of England, is never visited byinsects, and consequently, from its peculiar structure, never can seta seed. Many of our orchidaceous plants absolutely require the visitsof moths to remove their pollen-masses and thus to fertilise them. Ihave, also, reason to believe that humble-bees are indispensable tothe fertilisation of the heartsease (Viola tricolor), for other beesdo not visit this flower. From experiments which I have tried, I havefound that the visits of bees, if not indispensable, are at leasthighly beneficial to the fertilisation of our clovers; but humble-beesalone visit the common red clover (Trifolium pratense), as other beescannot reach the nectar. Hence I have very little doubt, that if thewhole genus of humble-bees became extinct or very rare in England, theheartsease and red clover would become very rare, or wholly disappear.The number of humble-bees in any district depends in a great degree onthe number of field-mice, which destroy their combs and nests; and Mr.H. Newman, who has long attended to the habits of humble-bees,believes that "more than two thirds of them are thus destroyed allover England." Now the number of mice is largely dependent, as everyone knows, on the number of cats; and Mr. Newman says, "Near villagesand small towns I have found the nests of humble-bees more numerousthan elsewhere, which I attribute to the number of cats that destroythe mice." Hence it is quite credible that the presence of a felineanimal in large numbers in a district might determine, through theintervention first of mice and then of bees, the frequency of certainflowers in that district!
The dependency of one organic being on another, as of a parasite onits prey, lies generally between beings remote in the scale of nature.This is often the case with those which may strictly be said tostruggle with each other for existence, as in the case of locusts andgrass-feeding quadrupeds. But the struggle almost invariably will bemost severe between the individuals of the same species, for theyfrequent the same districts, require the same food, and are exposed tothe same dangers. In the case of varieties of the same species, thestruggle will generally be almost equally severe, and we sometimes seethe contest soon decided: for instance, if several varieties of wheatbe sown together, and the mixed seed be resown, some of the varietieswhich best suit the soil or climate, or are naturally the mostfertile, will beat the others and so yield more seed, and willconsequently in a few years quite supplant the other varieties. Tokeep up a mixed stock of even such extremely close varieties as thevariously coloured sweet-peas, they must be each year harvestedseparately, and the seed then mixed in due proportion, otherwise theweaker kinds will steadily decrease in numbers and disappear. So againwith the varieties of sheep: it has been asserted that certainmountain-varieties will starve out other mountain-varieties, so thatthey cannot be kept together. The same result has followed fromkeeping together different varieties of the medicinal leech. It mayeven be doubted whether the varieties of any one of our domesticplants or animals have so exactly the same strength, habits, andconstitution, that the original proportions of a mixed stock could bekept up for half a dozen generations, if they were allowed to struggletogether, like beings in a state of nature, and if the seed or youngwere not annually sorted.
As species of the same genus have usually, though by no meansinvariably, some similarity in habits and constitution, and always instructure, the struggle will generally be more severe between speciesof the same genus, when they come into competition with each other,than between species of distinct genera. We see this in the recentextension over parts of the United States of one species of swallowhaving caused the decrease of another species. The recent increase ofthe missel-thrush in parts of Scotland has caused the decrease of thesong-thrush. How frequently we hear of one species of rat taking theplace of another species under the most different climates! In Russiathe small Asiatic cockroach has everywhere driven before it its greatcongener. One species of charlock will supplant another, and so inother cases. We can dimly see why the competition should be mostsevere between allied forms, which fill nearly the same place in theeconomy of nature; but probably in no one case could we precisely saywhy one species has been victorious over another in the great battleof life.
A corollary of the highest importance may be deduced from theforegoing remarks, namely, that the structure of every organic beingis related, in the most essential yet often hidden manner, to that ofall other organic beings, with which it comes into competition forfood or residence, or from which it has to escape, or on which itpreys. This is obvious in the structure of the teeth and talons of thetiger; and in that of the legs and claws of the parasite which clingsto the hair on the tiger's body. But in the beautifully plumed seed ofthe dandelion, and in the flattened and fringed legs of thewater-beetle, the relation seems at first confined to the elements ofair and water. Yet the advantage of plumed seeds no doubt stands inthe closest relation to the land being already thickly clothed byother plants; so that the seeds may be widely distributed and fall onunoccupied ground. In the water-beetle, the structure of its legs, sowell adapted for diving, allows it to compete with other aquaticinsects, to hunt for its own prey, and to escape serving as prey toother animals.
The store of nutriment laid up within the seeds of many plants seemsat first sight to have no sort of relation to other plants. But fromthe strong growth of young plants produced from such seeds (as peasand beans), when sown in the midst of long grass, I suspect that thechief use of the nutriment in the seed is to favour the growth of theyoung seedling, whilst struggling with other plants growing vigorouslyall around.
Look at a plant in the midst of its range, why does it not double orquadruple its numbers? We know that it can perfectly well withstand alittle more heat or cold, dampness or dryness, for elsewhere it rangesinto slightly hotter or colder, damper or drier districts. In thiscase we can clearly see that if we wished in imagination to give theplant the power of increasing in number, we should have to give itsome advantage over its competitors, or over the animals which preyedon it. On the confines of its geographical range, a change ofconstitution with respect to climate would clearly be an advantage toour plant; but we have reason to believe that only a few plants oranimals range so far, that they are destroyed by the rigour of theclimate alone. Not until we reach the extreme confines of life, in thearctic regions or on the borders of an utter desert, will competitioncease. The land may be extremely cold or dry, yet there will becompetition between some few species, or between the individuals ofthe same species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a newcountry amongst new competitors, though the climate may be exactly thesame as in its former home, yet the conditions of its life willgenerally be changed in an essential manner. If we wished to increaseits average numbers in its new home, we should have to modify it in adifferent way to what we should have done in its native country; forwe should have to give it some advantage over a different set ofcompetitors or enemies.
It is good thus to try in our imagination to give any form someadvantage over another. Probably in no single instance should we knowwhat to do, so as to succeed. It will convince us of our ignorance onthe mutual relations of all organic beings; a conviction as necessary,as it seems to be difficult to acquire. All that we can do, is to keepsteadily in mind that each organic being is striving to increase at ageometrical ratio; that each at some period of its life, during someseason of the year, during each generation or at intervals, has tostruggle for life, and to suffer great destruction. When we reflect onthis struggle, we may console ourselves with the full belief, that thewar of nature is not incessant, that no fear is felt, that death isgenerally prompt, and that the vigorous, the healthy, and the happysurvive and multiply.