Imágenes de páginas
PDF
EPUB

most obscure. Look at the most vigorous destruction of vermin. If not one head species; by as much as it swarms in num- of game were shot during the next twenty bers, by so much will it tend to increase years in England, and, at the same time, still further. We know not exactly what if no vermin were destroyed, there would, the checks are even in a single instance. in all probability, be less game than at Nor will this surprise any one who re- present, although hundreds of thousands flects how ignorant we are on this head, of game animals are now annually shot. even in regard to mankind, although so On the other hand, in some cases, as with incomparably better known than any the elephant, none are destroyed by beasts other animal. This subject of the checks of prey; for even the tiger in India most to increase has been ably treated by sev- rarely dares to attack a young elephant eral authors, and I hope in a future work protected by its dam. to discuss it at considerable length, more Climate plays an important part in deespecially in regard to the feral animals termining the average numbers of a speof South America. Here I will make cies, and periodical seasons of extreme only a few remarks, just to recall to the cold or drought seem to be the most reader's mind some of the chief points. effective of all checks. I estimated Eggs or very young animals seem gener- (chiefly from the greatly reduced numally to suffer most, but this is not invari- bers of nests in the spring) that the winably the case. With plants there is a vast ter of 1854-55 destroyed four-fifths of the destruction of seeds, but, from some ob- birds in my own grounds; and this is a servations which I have made, it appears tremendous destruction, when we rememthat the seedlings suffer most from ger- ber that ten per cent. is an extraordinarily minating in ground already thickly severe mortality from epidemics with stocked with other plants. Seedlings, man.

The action of climate seems at also, are destroyed in vast numbers by first sight to be quite independent of the various enemies; for instance, on a piece struggle for existence; but in so far as of ground three feet long and two wide, climate chiefly acts in reducing food, it dug and cleared, and where there could brings on the most severe struggle bebe no choking from other plants, I marked tween the individuals, whether of the all the seedlings of our native weeds as same or of distinct species, which subsist they came up, and out of 357 no less on the same kind of food. Even when than 295 were destroyed, chiefly by slugs climate, for instance extreme cold, acts and insects. If turf which has long been directly, it will be the least vigorous indimown, and the case would be the same viduals, or those which have got least with turf closely browsed by quadrupeds, food through the advancing winter, which be let to grow, the more vigorous plants will suffer most. When we travel from gradually kill the less vigorous, though south to north, or from a damp region to fully grown plants; thus out of twenty a dry, we invariably see some species species growing on a little plot of mown gradually getting rarer and rarer, and turf (three feet by four) nine species finally disappearing; and the change of perished, from the other species being al- climate being conspicuous, we are tempted lowed to grow up freely.

to attribute the whole effect to its direct The amount of food for each species action. But this is a false view; we forof course gives the extreme limit to which get that each species, even where it most each can increase; but very frequently it abounds, is constantly suffering enormous is not the obtaining food, but the serving destruction at some period of its life, from as prey to other animals, which deter- enemies or from competitors for the same mines the average numbers of a species. place and food; and if these enemies or Thus, there seems to be little doubt that competitors be in the least degree favored the stock of partridges, grouse and hares by any slight change of climate, they will on any large estate depends chiefly on the increase in numbers; and as each area is already fully stocked with inhabitants, tionately to the supply of seed, as their the other species must decrease. When numbers are checked during winter; but we travel southward and see a species de- any one who has tried, knows how troucreasing in numbers, we may feel sure blesome it is to get seed from a few wheat that the cause lies quite as much in other or other such plants in a garden: I have species being favored as in this one being in this case lost every single seed. This hurt. So it is when we travel northward, view of the necessity of a large stock of but in a somewhat lesser degree, for the the same species for its preservation, exnumber of species of all kinds, and there- plains, I believe, some singular facts in fore of competitors, decreases northward; nature, such as that of very rare plants hence in going northward, or in ascending being sometimes extremely abundant, in a mountain, we far oftener meet with the few spots where they do exist; and stunted forms, due to the directly injuri- that of some social plants being social, ous action of climate, than we do in pro- that is abounding in individuals, even on ceeding southward or in descending a the extreme verge of their range. For in mountain. When we reach the Arctic such cases, we may believe, that a plant regions or snow-capped summits, or abso- could exist only where the conditions of lute deserts, the struggle for life is almost its life were so favorable that many could exclusively with the elements.

exist together, and thus save the species That climate acts in main part in- from utter destruction. I should add directly by favoring other species we that the good effects of intercrossing, and clearly see in the prodigious number of the ill effects of close interbreeding, no plants which in our gardens can perfectly doubt come into play in many of these well endure our climate, but which never cases; but I will not here enlarge on this become naturalized, for they cannot com- subject. pete with our native plants nor resist destruction by our native animals.

Complex Relations of All Animals and When a species, owing to highly favor

Plants to Each Other in the able circumstances, increases inordinately in numbers in a small tract, epidemics

Struggle for Existence at least, this seems generally to occur Many cases are on record showing how with our game animals—often ensue; and complex and unexpected are the checks here we have a limiting check independ- and relations between organic beings, ent of the struggle for life. But even which have to struggle together in the some of these so-called epidemics appear same country. I will give only a single to be due to parasitic worms, which have instance, which, though a simple one, infrom some cause, possibly in part through terested me. In Staffordshire, on the esfacility of diffusion among the crowded tate of a relation, where I had ample animals, been disproportionately favored : means of investigation, there was a large and here comes in a sort of struggle be- and extremely barren heath, which had tween the parasite and its prey.

never been touched by the hand of man; On the other hand, in many cases, a

but several hundred acres of exactly the large stock of individuals of the same spe- same nature had been inclosed twenty-five cies, relatively to the numbers of its ene- years previously and planted with Scotch mies, is absolutely necessary for its preser- fir. The change in the native vegetation vation. Thus we can easily raise plenty of the planted part of the heath was most of corn and rapeseed, etc., in our fields, remarkable, more than is generally seen because the seeds are in great excess com- in passing from one quite different soil pared with the number of birds which to another: not only the proportional feed on them; nor can the birds, though numbers of the heath-plants were wholly having a superabundance of food at this changed, but twelve species of plants (not one season, increase in number propor- | counting grasses and carices) flourished

in the plantations, which could not be horses nor dogs have ever run wild, found on the heath. The effect on the though they swarm southward and northinsects must have been still greater, for ward in a feral state; and Azara and six insectivorous birds were very common Rengger have shown that this is caused by in the plantations, which were not to be the greater number in Paraguay of a cerseen on the heath; and the heath was fre- tain fly, which lays its eggs in the navels quented by two or three distinct insectiv- of these animals when first born. The orous birds. Here we see how potent increase of these fies, numerous as they has been the effect of the introduction of are, must be habitually checked by some a single tree, nothing whatever else hav- means, probably by other parasitic ining been done, with the exception of the sects. Hence, if certain insectivorous land having been inclosed, so that cattle birds were to decrease in Paraguay, the could not enter. But how important an parasitic insects would probably increase; element inclosure is, I plainly saw near and this would lessen the number of the Farnham, in Surrey. Here there are navel-frequenting Aies—then cattle and extensive heaths, with a few clumps of horses would become feral, and this old Scotch firs on the distant hilltops: would certainly greatly alter (as indeed within the last ten years large spaces have I have observed in parts of South Amerbeen inclosed, and self-sown firs are now ica) the vegetation: this again would springing up in multitudes, so close to- largely affect the insects; and this, as we gether that all cannot live. When I as- have just seen in Staffordshire, the incertained that these young trees had not sectivorous birds, and so onward in everbeen sown or planted, I was so much sur- increasing circles of complexity. Not prised at their numbers that I went to that under nature the relations will ever several points of view, whence I could ex- be as simple as this. Battle within batamine hundreds of acres of the uninclosed tle must be continually recurring with heath, and literally I could not see a sin- varying success; and yet in the long run gle Scotch fir, except the old planted the forces are so nicely balanced that the clumps. But on looking closely between face of nature remains for long periods the stems of the heath, I found a multi- of time uniform, though assuredly the tude of seedlings and little trees which merest trifle would give the victory to had been perpetually browsed down by one organic being over another. Neverthe cattle. In one square yard, at a point theless, so profound is our ignorance, and some hundred yards distant from one of so high our presumption, that we marvel the old clumps, I counted thirty-two lit- when we hear of the extinction of an ortle trees; and one of them, with twenty- ganic being; and as we do not see the six rings of growth, had, during many cause, we invoke cataclysms to desolate years, tried to raise its head above the the world, or invent laws on the duration stems of the heath, and had failed. No of the forms of life! wonder that, as soon as the land was inclosed, it had became thickly clothed with I am tempted to give one more instance vigorously growing young firs. Yet the showing how plants and animals, remote heath was so extremely barren and so in the scale of nature, are bound together extensive that no one would ever have by a web of complex relations. I shall imagined that cattle would have so closely hereafter have occasion show that the and effectually searched it for food. exotic Lobelia fulgens is never visited in

Here we see that cattle absolutely de- my garden by insects, and consequently, termine the existence of the Scotch fir; from its peculiar structure, never sets a but in several parts of the world insects seed. Nearly all our orchidaceous plants determine the existence of cattle. Per- absolutely require the visits of insects to haps Paraguay offers the most curious in- remove their pollen-masses and thus to stance of this; for here neither cattle nor fertilize them. I find from experiments

that humble-bees are almost indispensa- existence of the species. In some cases it ble to the fertilization of the heart's-ease can be shown that widely-different checks (Viola tricolor), for other bees do not act on the same species in different disvisit this flower. I have also found that tricts. When we look at the plants and the visits of bees are necessary for the bushes clothing an entangled bank, we of ;

to instance, 20 heads of Dutch clover (Tri- tional numbers and kinds to what we call folium repens) yielded 2,290 seeds, but chance. But how false a view is this! 20 other heads protected from bees pro- Every one has heard that when an Amerduced not one. Again, 100 heads of red ican forest is cut down, a very different clover (T. pratense) produced 2,700 vegetation springs up; but it has been seeds, but the same number of protected observed that ancient Indian ruins in the heads produced not a single seed. Hum- Southern United States, which must ble-bees alone visit red clover, as other formerly have been cleared of trees, now bees cannot reach the nectar. It has been display the same beautiful diversity and suggested that moths may fertilize the proportion of kinds as in the surrounding clovers; but I doubt whether they could virgin forest. What a struggle must do so in the case of the red clover, from have gone on during long centuries betheir weight not being sufficient to de- tween the several kinds of trees, each press the wing petals. Hence we may in- annually scattering its seeds by the thoufer as highly probable that, if the whole sand; what war between insect and insect genus of humble-bees became extinct or -between insects, snails, and other anivery rare in England, the heart's-ease and mals with birds and beasts of prey-all red clover would become very rare, or striving to increase, all feeding on each wholly disappear. The number of hum-other, or on the trees, their seeds and ble-bees in any district depends in a great seedlings, or on the other plants which measure upon the number of field-mice, first clothed the ground and thus checked which destroy their combs and nests; and the growth of the trees! Throw up a Col. Newman, who has long attended to handful of feathers, and all fall to the the habits of humble-bees, believes that ground according to definite laws; but "more than two-thirds of them are thus how simple is the problem where each destroyed all over England.” Now the shall fall compared to that of the action number of mice is largely dependent, as and reaction of the innumerable plants every one knows, on the number of cats; and animals which have determined, in and Col. Newman says, “Near villages the course of centuries, the proportional and small towns I have found the nests numbers and kinds of trees now growing of humble-bees more numerous than on the old Indian ruins ! elsewhere, which I attribute to the num- The dependency of one organic being ber of cats that destroy the mice.” Hence on another, as of a parasite on its prey, it is quite credible that the presence of a lies generally between beings remote in feline animal in large numbers in a dis- the scale of nature. This is likewise trict might determine, through the inter- sometimes the case with those which may vention first of mice and then of bees, the be strictly said to struggle with each other frequency of certain flowers in that dis- for existence, as in the case of locusts and trict !

grass-feeding quadrupeds. But the strugIn the case of every species, many dif- gle will almost invariably be most severe ferent checks, acting at different periods between the individuals of the same speof life, and during different seasons or cies, for they frequent the same districts, years, probably come into play; some one require the same food, and are exposed to check or some few being generally the the same dangers. In the case of varimost potent; but all will concur in de- eties of the same species, the struggle will termining the average number or even the generally be almost equally severe, and we sometimes see the contest soon decided : of one species of rat taking the place of for instance, if several varieties of wheat another species under the most different be sown together, and the mixed seed be climates! In Russia the small Asiatic resown, some of the varieties which best cockroach has everywhere driven before it suit the soil or climate, or are naturally its great congener. In Australia the imthe most fertile, will beat the others and ported hive-bee is rapidly exterminating so yield more seed, and will consequently the small, stingless native bee. One spein a few years supplant the other vari- cies of charlock has been known to supeties. To keep up a mixed stock of even plant another species; and so in other such extremely close varieties as the vari- cases. We can dimly see why the comously-colored sweet peas, they must be petition should be most severe between each year harvested separately, and the allied forms, which fill nearly the same seed then mixed in due proportion, other- place in the economy of nature; but wise the weaker kinds will steadily de- probably in no one case could we precrease in number and disappear. So cisely say why one species has been vicagain with the varieties of sheep it has torious over another in the great battle of been asserted that certain mountain- life. varieties will starve out other mountain- A corollary of the highest importance varieties, so that they cannot be kept may be deduced from the foregoing retogether. The same result has followed marks; namely, that the structure of from keeping together different varieties every organic being is related, in the most of the medicinal leech. It may even be essential yet often hidden manner, to doubted whether the varieties of any of that of all the other organic beings, with our domestic plants or animals have so which it comes into competition for food exactly the same strength, habits, and or residence, or from which it has to esconstitution, that the original proportions cape, or on which it preys. This is obviof a mixed stock (crossing being pre- ous in the structure of the teeth and vented) could be kept up for half a dozen talons of the tiger; and in that of the legs generations, if they were allowed to strug- and claws of the parasite which clings to gle together, in the same manner as be- the hair on the tiger's body. But in the ings in a state of nature, and if the seed beautifully plumed seed of the dandelion, or young were not annually preserved in and in the flattened and fringed legs of due proportion.

the water-beetle, the relation seems at

first confined to the elements of air and Struggle for Life Most Severe Between

water. Yet the advantage of plumed Individuals and Varieties of the

seeds no doubt stands in the closest relaSame Species

tion to the land being already thickly

clothed with other plants; so that the As the species of the same genus usu- seeds may be widely distributed and fall ally have, though by no means invariably, on unoccupied ground. In the watermuch similarity in habits and constitution, beetle, the structure of its legs so well and always in structure, the struggle will adapted for diving, allows it to compete generally be more severe between them, if with other aquatic insects, to hunt for its they come into competition with each own prey, and to escape serving as prey other, than between the species of distinct to other animals. genera. We see this in the recent exten- The store of nutriment laid up within sion over parts of the United States of the seeds of many plants seems at first one species of swallow having caused the sight to have no sort of relation to other decrease of another species. The recent plants. But from the strong growth of increase of the missel-thrush in parts of young plants produced from such seeds, Scotland has caused the decrease of the as peas and beans, when sown in the midst song-thrush. How frequently we hear l of long grass, it may be suspected that the

« AnteriorContinuar »