But the dispersal of insects requires to be looked at from another point of view. They are, of all animals, perhaps the most wonderfully adapted for special conditions; and are so often fitted to fill one place in nature and one only, that the barriers against their permanent displacement are almost as numerous and as effective as their means of dispersal. Hundreds of species of lepidoptera, for example, can subsist in the larva state only on one species of plant; so that even if the perfect insects were carried to a new country, the continuance of the race would depend upon the same or a closely allied plant being abundant there. Other insects require succulent vegetable food all the year round, and are therefore confined to tropical regions; some can live only in deserts, others in forests; some are dependent on water-plants, some on mountain-vegetation. Many are so intimately connected with other insects during some part of their existence that they could not live without them; such are the parasitical hymenoptera and diptera, and those mimicking species whose welfare depends upon their being {34}mistaken for something else. Then again, insects have enemies in every stage of their existence--the egg, the larva, the pupa, and the perfect form; and the abundance of any one of these enemies may render their survival impossible in a country otherwise well suited to them. Ever bearing in mind these two opposing classes of facts, we shall not be surprised at the enormous range of some groups of insects, and at the extreme localization of others; and shall be able to give a rational account of many phenomena of distribution that would otherwise seem quite unintelligible. {35}CHAPTER III. DISTRIBUTION AS AFFECTED BY THE CONDITIONS AND CHANGES OF THE EARTH'S SURFACE. The distribution of animals over the earth's surface, is evidently dependent in great measure upon those grand and important characteristics of our globe, the study of which is termed physical geography. The proportion of land and water; the outlines and distribution of continents; the depth of seas and oceans; the position of islands; the height, direction, and continuity of mountain chains; the position and extent of deserts, lakes, and forests; the direction and velocity of ocean currents, as well as of prevalent winds and hurricanes; and lastly, the distribution of heat and cold, of rain, snow, and ice, both in their means and in their extremes, have all to be considered when we endeavour to account for the often unequal and unsymmetrical manner in which animals are dispersed over the globe. But even this knowledge is insufficient unless we inquire further as to the evidence of permanence possessed by each of these features, in order that we may give due weight to the various causes that have led to the existing facts of animal distribution.