Continental limitation is effected by mountain barriers. Thus, according to Mr. Wallace, almost all the mammalia, birds, and insects on one side of the Andes and Rocky Mountains are distinct in species from those on the other; while a similar difference, but smaller in degree, exists with reference to regions adjacent to the Alps and Pyrenees.
Climate, broad rivers, seas, oceans, forests, and even large desert wastes, like the Sahara or the great desert of Gobi, also act more or less effectively as girdles which confine species within certain limits.
Dependence on each other or on supplies of appropriate food also form minor yet practical factors in the sum of limitation; and a curious example of the first is given by Dr. Van Lennep with reference to the small migratory birds that are unable to perform the flight of three hundred and fifty miles across the Mediterranean. He states that these are carried across on the backs of cranes.[39]
In the autumn many flocks of cranes may be seen coming from the North, with the first cold blast from that quarter, flying low, and uttering a peculiar cry, as if of alarm, as they circle over the cultivated plains. Little birds of every species may be seen flying up to them, while the twittering cries of those already comfortably settled upon their backs may be distinctly heard. On their return in the spring they fly high, apparently considering that their little passengers can easily find their way down to the earth.
The question of food-supply is involved in the more extended subject of geological structure, as controlling the flora and the insect life dependent on it. As an example we may cite the disappearance of the capercailzie from Denmark with the decay of the pine forests abundant during late Tertiary periods.
Collision, direct or indirect, with inimical species often has a fatal ending. Thus the dodo was exterminated by the swine which the early visitors introduced to the Mauritius and permitted to run wild there; while the indigenous insects, mollusca, and perhaps some of the birds of St. Helena, disappeared as soon as the introduction of goats caused the destruction of the whole flora of forest trees.
The Tsetse fly extirpates all horses, dogs, and cattle, from certain districts of South Africa, and a representative species in Paraguay is equally fatal to new-born cattle and horses.
Mr. Darwin[40] shows that the struggle is more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera. Thus one species of swallow has recently expelled another from part of the United States; and the missel-thrush has driven the song-thrush from part of Scotland. In Australia the imported hive-bee is rapidly exterminating the small stingless native bee, and similar cases might be found in any number.
Mr. Wallace, in quoting Mr. Darwin as to these facts, points the conclusion that “any slight change, therefore, of physical geography or of climate, which allows allied species hitherto inhabiting distinct areas to come into contact, will often lead to the extermination of one of them.”
It is the province of the palæontologist to enumerate the many remarkable forms which have passed away since man’s first appearance upon the globe, and to trace their fluctuations over both hemispheres as determined by the advance and retreat of glacial conditions, and by the protean forms assumed by past and existing continents under oscillations of elevation and depression. Many interesting points, such as the dates of the successive separation of Ireland and Great Britain from the main continent, can be determined with accuracy from the record furnished by the fossil remains of animals of those times; and many interesting associations of animals with man at various dates, in our present island home and in other countries, have been traced by the discovery of their remains in connection with his, in bone deposits in caverns and elsewhere.