Divergence of character has a double purpose and use. In the first place it enables a species which is being overcome by rivals, or is in process of extinction by enemies, to save itself by adopting new habits or by occupying vacant places in nature. This is the immediate and obvious effect of all the numerous examples of divergence of character which we have pointed out. But there is another and less obvious result, which is, that the greater the diversity in the organisms inhabiting a country or district the greater will be the total amount of life that can be supported there. Hence the continued action of the struggle for existence will tend to bring about more and more diversity in each area, which may be shown to be the case by several kinds of evidence. As an example, a piece of turf, three feet by four in size, was found by Mr. Darwin to contain twenty species of plants, and these twenty species belonged to eighteen genera and to eight orders, showing how greatly they differed from each other. Farmers find that a greater quantity of hay is obtained from ground sown with a variety of genera of grasses, clover, etc., than from similar land sown with one or two species only; and the same principle applies to rotation of crops, plants differing very widely from each other giving the best results. So, in small and uniform islands, and in small ponds of fresh water, the plants and insects, though few in number, are found to be wonderfully varied in character.
The same principle is seen in the naturalisation of plants and animals by man's agency in distant lands, for the species that thrive best and establish themselves permanently are not only very varied among themselves but differ greatly from the native inhabitants. Thus, in the Northern United States there are, according to Dr. Asa Gray, 260 naturalised flowering plants which belong to no less than 162 genera; and of these, 100 genera are not natives of the United States. So, in Australia, the rabbit, though totally unlike any native animal, has increased so much that it probably outnumbers in individuals all the native mammals of the country; and in New Zealand the rabbit and the pig have equally multiplied. Darwin remarks that this "advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same body. No physiologist doubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws more nutriment from these substances. So, in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves."[39]
The most closely allied Species inhabit distinct Areas.
One of the curious results of the general action of this principle in nature is, that the most closely allied species—those whose differences though often real and important are hardly perceptible to any one but a naturalist—are usually not found in the same but in widely separated countries. Thus, the nearest allies to our European golden plover are found in North America and East Asia; the nearest ally of our European jay is found in Japan, although there are several other species of jays in Western Asia and North Africa; and though we have several species of titmice in England they are not very closely allied to each other. The form most akin to our blue tit is the azure tit of Central Asia (Parus azureus); the Parus ledouci of Algeria is very near our coal tit, and the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to our marsh tit. So, our four species of wild pigeons—the ring-dove, stock-dove, rock-pigeon, and turtle-dove—are not closely allied to each other, but each of them belongs, according to some ornithologists, to a separate genus or subgenus, and has its nearest relatives in distant parts of Asia and Africa. In mammalia the same thing occurs. Each mountain region of Europe and Asia has usually its own species of wild sheep and goat, and sometimes of antelope and deer; so that in each region there is found the greatest diversity in this class of animals, while the closest allies inhabit quite distinct and often distant areas. In plants we find the same phenomenon prevalent. Distinct species of columbine are found in Central Europe (Aguilegia vulgaris), in Eastern Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina), in the Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in Corsica (A. Bernardi), but rarely are two species found in the same area. So, each part of the world has its own peculiar forms of pines, firs, and cedars, but the closely allied species or varieties are in almost every case inhabitants of distinct areas. Examples are the deodar of the Himalayas, the cedar of Lebanon, and that of North Africa, all very closely allied but confined to distinct areas; and the numerous closely allied species of true pine (genus Pinus), which almost always inhabit different countries or occupy different stations. We will now consider some other modes in which natural selection will act, to adapt organisms to changed conditions.
Adaptation to Conditions at Various Periods of Life.
It is found, that, in domestic animals and cultivated plants, variations occurring at any one period of life reappear in the offspring at the same period, and can be perpetuated and increased by selection without modifying other parts of the organisation. Thus, variations in the caterpillar or the cocoon of the silkworm, in the eggs of poultry, and in the seeds or young shoots of many culinary vegetables, have been accumulated till those parts have become greatly modified and, for man's purposes, improved. Owing to this fact it is easy for organisms to become so modified as to avoid dangers that occur at any one period of life. Thus it is that so many seeds have become adapted to various modes of dissemination or protection. Some are winged, or have down or hairs attached to them, so as to enable them to be carried long distances in the air; others have curious hooks and prickles, which cause them to be attached firmly to the fur of mammals or the feathers of birds; while others are buried within sweet or juicy and brightly coloured fruits, which are seen and devoured by birds, the hard smooth seeds passing through their bodies in a fit state for germination. In the struggle for existence it must benefit a plant to have increased means of dispersing its seeds, and of thus having young plants produced in a greater variety of soils, aspects, and surroundings, with a greater chance of some of them escaping their numerous enemies and arriving at maturity. The various differences referred to would, therefore, be brought about by variation and survival of the fittest, just as surely as the length and quality of cotton on the seed of the cotton-plant have been increased by man's selection.
The larvae of insects have thus been wonderfully modified in order to escape the numerous enemies to whose attacks they are exposed at this period of their existence. Their colours and markings have become marvellously adapted to conceal them among the foliage of the plant they live upon, and this colour often changes completely after the last moult, when the creature has to descend to the ground for its change to the pupa state, during which period a brown instead of a green colour is protective. Others have acquired curious attitudes and large ocelli, which cause them to resemble the head of some reptile, or they have curious horns or coloured ejectile processes which frighten away enemies; while a great number have acquired secretions which render them offensive to the taste of their enemies, and these are always adorned with very conspicuous markings or brilliant colours, which serve as a sign of inedibility and prevent their being needlessly attacked. This, however, is a portion of the very large subject of organic colour and marking, which will be fully discussed and illustrated in a separate chapter.
In this way every possible modification of an animal or plant, whether in colour, form, structure, or habits, which would be serviceable to it or to its progeny at any period of its existence, may be readily brought about. There are some curious organs which are used only once in a creature's life, but which are yet essential to its existence, and thus have very much the appearance of design by an intelligent designer. Such are, the great jaws possessed by some insects, used exclusively for opening the cocoon, and the hard tip to the beak of unhatched birds used for breaking the eggshell. The increase in thickness or hardness of the cocoons or the eggs being useful for protection against enemies or to avoid accidents, it is probable that the change has been very gradual, because it would be constantly checked by the necessity for a corresponding change in the young insects or birds enabling them to overcome the additional obstacle of a tougher cocoon or a harder eggshell. As we have seen, however, that every part of the organism appears to be varying independently, at the same time, though to different amounts, there seems no reason to believe that the necessity for two or more coincident variations would prevent the required change from taking place.
The Continued Existence of Low Forms of Life.
Since species are continually undergoing modifications giving them some superiority over other species or enabling them to occupy fresh places in nature, it may be asked—Why do any low forms continue to exist? Why have they not long since been improved and developed into higher forms? The answer, probably, is, that these low forms occupy places in nature which cannot be filled by higher forms, and that they have few or no competitors; they therefore continue to exist. Thus, earthworms are adapted to their mode of life better than they would be if more highly organised. So, in the ocean, the minute foraminifera and infusoria, and the larger sponges and corals, occupy places which more highly developed creatures could not fill. They form, as it were, the base of the great structure of animal life, on which the next higher forms rest; and though in the course of ages they may undergo some changes, and diversification of form and structure, in accordance with changed conditions, their essential nature has probably remained the same from the very dawn of life on the earth. The low aquatic diatomaceae and confervae, together with the lowest fungi and lichens, occupy a similar position in the vegetable kingdom, filling places in nature which would be left vacant if only highly organised plants existed. There is, therefore, no motive power to destroy or seriously to modify them; and they have thus probably persisted, under slightly varying forms, through all geological time.