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enemy in the forces of inorganic nature. Each species can sustain a certain amount of heat and cold, each requires a certain amount of moisture at the right season, each wants a proper amount of light or of direct sunshine, each needs certain elements in the soil; the failure of a due proportion in these inorganic conditions causes weakness, and thus leads to speedy death. The struggle for existence in plants is, therefore, threefold in character and infinite in complexity, and the result is seen in their curiously irregular distribution over the face of the earth. Not only has each country its distinct plants, but every valley, every hillside, almost every hedgerow, has a different set of plants from its adjacent valley, hillside, or hedgerow—if not always different in the actual species yet very different in comparative abundance, some which are rare in the one being common in the other. Hence it happens that slight changes of conditions often produce great changes in the flora of a country. Thus in 1740 and the two following years the larva of a moth (Charæas graminis) committed such destruction in many of the meadows of Sweden that the grass was greatly diminished in quantity, and many plants which were before choked by the grass sprang up, and the ground became variegated with a multitude of different species of flowers. The introduction of goats into the island of St. Helena led to the entire destruction of the native forests, consisting of about a hundred distinct species of trees and shrubs, the young plants being devoured by the goats as fast as they grew up. The camel is a still greater enemy to woody vegetation than the goat, and Mr. Marsh believes that forests would soon cover considerable tracts of the Arabian and African deserts if the goat and the camel were removed from them. Even in many parts of our own country the existence of trees is dependent on the absence of cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in Surrey, a few clumps of old Scotch firs, but no young trees over hundreds of acres. Some portions of the heath had, however, been enclosed a few years before, and these enclosures were crowded with young fir-trees growing too close together for all to live ; and these were not sown or planted, nothing having been done to the ground beyond enclosing it
1 The Earth as Modified by Human Action, p. 51.
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so as to keep out cattle. On ascertaining this, Mr. Darwin was so much surprised that he searched among the heather in the unenclosed parts, and there he found multitudes of little trees and seedlings which had been perpetually browsed down by the cattle. In one square yard, at a point about a hundred yards from one of the old clumps of firs, he counted thirtytwo little trees, and one of them had twenty-six rings of growth, showing that it had for many years tried to raise its head above the stems of the heather and had failed. Yet this heath was very extensive and very barren, and, as Mr. Darwin remarks, no one would ever have imagined that cattle would have so closely and so effectually searched it for food.
In the case of animals, the competition and struggle are more obvious. The vegetation of a given district can only support a certain number of animals, and the different kinds of plant-eaters will compete together for it. They will also have insects for their competitors, and these insects will be kept down by birds, which will thus assist the mammalia. But there will also be carnivora destroying the herbivora; while small rodents, like the lemming and some of the fieldmice, often destroy so much vegetation as materially to affect the food of all the other groups of animals. Droughts, floods, severe winters, storms and hurricanes will injure these in various degrees, but no one species can be diminished in numbers without the effect being felt in various complex ways by all the rest. A few illustrations of this reciprocal action must be given.
Illustrative Cases of the Struggle for Life. Sir Charles Lyell observes that if, by the attacks of seals or other marine foes, salmon are reduced in numbers, the consequence will be that otters, living far inland, will be deprived of food and will then destroy many young birds or quadrupeds, so that the increase of a marine animal may cause the destruction of many land animals hundreds of miles away. Mr. Darwin carefully observed the effects produced by planting a few hundred acres of Scotch fir, in Staffordshire, on part of a very extensive heath which had never been cultivated. After the planted portion was about twenty-five years old he observed that the change in the native vegetation was greater than is often seen in passing from one quite different soil to another. Besides a great change in the proportional numbers of the native heath-plants, twelve species which could not be found on the heath flourished in the plantations. The effect on the insect life must have been still greater, for six insectivorous birds which were very common in the plantations were not to be seen on the heath, which was, however, frequented by two or three different species of insectivorous birds. It would have required continued study for several years to determine all the differences in the organic life of the two areas, but the facts stated by Mr. Darwin are sufficient to show how great a change may be effected by the introduction of a single kind of tree and the keeping out of cattle. . The next case I will give in Mr. Darwin's own words: “In several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state ; and Azara and Rengger have shown that this is caused by the greater numbers, in Paraguay, of a certain fly which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navelfrequenting flies—then cattle and horses would become feral, and this would greatly alter (as indeed I have observed in parts of South America) the vegetation : this again would largely affect the insects, and this, as we have just seen in Staffordshire, the insectivorous birds, and so onward in everincreasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long run the forces are so nicely balanced, that the face of nature remains for a long time uniform, though assuredly the merest trifle would give the victory to one organic being over another.”1
Such cases as the above may perhaps be thought exceptional, but there is good reason to believe that they are by no means rare, but are illustrations of what is going on in every part of the world, only it is very difficult for us to trace out the complex reactions that are everywhere occurring. The general impression of the ordinary observer seems to be that wild animals and plants live peaceful lives and have few troubles, each being exactly suited to its place and surroundings, and therefore having no difficulty in maintaining itself. Before showing that this view is, everywhere and always, demonstrably untrue, we will consider one other case of the complex relations of distinct organisms adduced by Mr. Darwin, and often quoted for its striking and almost eccentric character. It is now well known that many flowers require to be fertilised by insects in order to produce seed, and this fertilisation can, in some cases, only be effected by one particular species of insect to which the flower has become specially adapted. Two of our common plants, the wild heart'sease (Viola tricolor) and the red clover (Trifolium pratense), are thus fertilised by humble-bees almost exclusively, and if these insects are prevented from visiting the flowers, they produce either no seed at all or exceedingly few. Now it is known that field-mice destroy the combs and nests of humble-bees, and Colonel Newman, who has paid great attention to these insects, believes that more than two-thirds of all the humble-bees' nests in England are thus destroyed. But the number of mice depends a good deal on the number of cats; and the same observer says that near villages and towns he has found the nests of humble-bees more numerous than elsewhere, which he attributes to the number of cats that destroy the mice. Hence it follows, that the abundance of red clover and wild heart's-ease in a district will depend on a good supply of cats to kill the mice, which would otherwise destroy and keep down the humble-bees and prevent them from fertilising the flowers. A chain of connection has thus been found between such totally distinct organisms as flesh-eating mammalia and sweetsmelling flowers, the abundance or scarcity of the one closely corresponding to that of the other !
The following account of the struggle between trees in the forests of Denmark, from the researches of M. Hansten
Blangsted, strikingly illustrates our subject. The chief combatants are the beech and the birch, the former being everywhere successful in its invasions. Forests composed wholly of birch are now only found in sterile, sandy tracts; everywhere else the trees are mixed, and wherever the soil is favourable the beech rapidly drives out the birch. The latter loses its branches at the touch of the beech, and devotes all its strength to the upper part where it towers above the beech. It may live long in this way, but it succumbs ultimately in the fight—of old age if of nothing else, for the life of the birch in Denmark is shorter than that of the beech. The writer believes that light (or rather shade) is the cause of the superiority of the latter, for it has a greater development of its branches than the birch, which is more open and thus allows the rays of the sun to pass through to the soil below, while the tufted, bushy top of the beech preserves a deep shade at its base. Hardly any young plants can grow under the beech except its own shoots; and while the beech can flourish under the shade of the birch, the latter dies immediately under the beech. The birch has only been saved from total extermination by the facts that it had possession of the Danish forests long before the beech ever reached the country, and that certain districts are unfavourable to the growth of the latter. But wherever the soil has been enriched by the decomposition of the leaves of the birch the battle begins. The birch still flourishes on the borders of lakes and other marshy places, where its enemy cannot exist. In the same way, in the forests of Zeeland, the fir forests are disappearing before the beech. Left to themselves, the firs are soon displaced by the beech. The struggle between the latter and the oak is longer and more stubborn, for the branches and foliage of the oak are thicker, and offer much resistance to the passage of light. The oak, also, has greater longevity; but, sooner or later, it too succumbs, because it cannot develop in the shadow of the beech. The earliest forests of Denmark were mainly composed of aspens, with which the birch was apparently associated ; gradually the soil was raised, and the climate grew milder; then the fir came and formed large forests. This tree ruled for centuries, and then ceded the
1 See Nature, vol. xxxi. p. 63.