of prey) were to increase in Paraguay, the flies would decrease,—then cattle and horses would become feral; and this would certainly 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 just have seen in Staffordshire, the insectivorous birds, and so onwards in ever-increasing circles of complexity."
In the struggle for life, the strongest live; or, in other words, those best fitted to live in the environment endure. Animals and plants produce in vast excess of the possibility of life. A destruction of life is going on to an almost incredible amount. Were this not the case, the slowest breeders in existence would soon cover the earth so as to occupy every inch of space. Darwin reckons that the elephant, the slowest breeder, if allowed to go on unchecked, and to live his allotted term of years, would in five centuries produce fifteen millions of elephants from one pair. If every cod's egg had developed into a full-grown fish, the whole ocean would, ages ago, have been packed with them, like herrings in a box. In this destruction, the weaker animals and plants—those least fitted to thrive under the influences around—become the prey of others better fitted for the struggle, or die of their own lack of assimilative force. Thus, through untold ages of shifting outward circumstances, the plastic forms of organic life have been remoulded. A little obscure plant, the food of an insect, dies out; the insect itself, deprived of its food, dies out or migrates; the bird which fed upon it dies out or migrates; the bird of prey follows the like course. Migration introduces them to an entirely new state of existence, temperature, food, and antagonists. The migrating animals are replaced by others, which likewise experience new surroundings; and thus the extirpation of a single plant may determine a long series of changes. Instances of this kind are not uncommon. What must these changes have been throughout the remote ages which have turned sea-shore mud into uplifted mountain chains, and sunk long-stretching, sunny hills into the ocean depths!
Darwin constructs his theory of gradual differentiation on the evidence thus obtained. He takes a given specific animal form, and supposes that, owing to some external change in a given locality, it takes on some correspondent variation. But all of the individuals of the species may not be likewise affected. The circumstances may alter in one place and not in another. The result will be two varieties of animal. The variety goes on increasing in diversity, while the original still continues to produce its like. By and by the variety, having a greater tendency to vary, from its having already done so, undergoes a new differentiation, the difference being, in all cases, slight, and the time between the periods of maximum change being hundreds, thousands of years. One of the new varieties may by peculiar circumstances take on a special amplitude of growth, while the other, peculiarly circumstanced, may be contracted and dwarfed. One of the original varieties may by this time have disappeared. The original itself may have disappeared. Thus the connecting link between the two forms is lost. The more individualized form may go on accenting its own peculiar characters, and again be broken into new varieties, some of which may retain the old characters in circumscribed areas, while others may increase in greater abundance and occupy a much wider area. The wider the field of life, the more numerous the differing influences and the more diverse the conditions the animal must undergo. Thence arise more differentiations. After the lapse of some millions of ages, these constantly forking growths will have taken on a diversity to which that of the pouters and fan-tails is trifling.
Some forms may be less plastic than others, and give way less readily to the incident forces. These may remain unchanged for a far longer period than subsequent varieties, and be coexistent
