This introduces the subject of the environmental distribution of animal and plant life, or the distribution of animale and plants according to the different kinds of environment in which they are found. Returning once again to the original imaginary example of how life developed and spread over the earth, it is not beyond the imagination to see how, by the application of the principles of the survival of the fittest and the adaptation to environmental changes, life might establish itself on this earth anywhere where conditions favourable to it might exist, that is to say, conditions not too hot or too cold or too dry. In the course of the foregoing survey of the geographical distribution of animal life it has been noted that they can and do exist in deserts, on plains and on mountains right up to and beyond the snow line, as well as in ponds, is streams, in rivers, in lakes, in the ocean from the shore-line down to the abyssal depths and out in the open sea. It has been shown how that there is an adaptation on the part of animals and plants to all kinds of conditions, and that animal and plant life is divisible into definite groups. There is a desert fauna, a forest fauna, a plains fauna, a mountain fauna, a shore fauna and a sea fauna. It may further be noted that within each of these divisions there are further sub-divisions.
The desert fauna is divided into a sand-dune fauna, a brackish lake or lagoon fauna and a rocky barrens fauna. In a forest there is a tree-top fauna, an underbrush fauna and a stream-side fauna. On a plain there is a copse fauna, a grassland or meadow fauna, a shoreland fauna and a marsh fauna. The same applies to plant life: but it is impossible here to go into further detail. All that can be said here is that life exists everywhere on the earth, that every sphere and every medium is occupied, not by one or two forms, but by dozens, hundreds or even thousands of different kinds of animals and plants.
Little has been said about animal and plant life in the air. This is because as yet no form has been able to take up its permanent quarters in the air. Every living creature that goes into the air or that uses the air as a medium for locomotion must return to earth or water, so that its distribution is really on land or in water, and it falls within one or another of the life-zones that have been mentioned. However, locomotion by air has a very considerable bearing upon the subject of animal distribution, as will readily be realized when some of the best flyers in the bird world are considered. Species that make the most use of air locomotion naturally come to have the widest distribution, as evidenced by the world-spanning Arctic tern, which summers in the Arctic and winters in the Antarctic; or the great black or cinereous vulture, whose range is roughly all over Europe, North Africa and Asia, a lonely bird that spends most of its time high up in the air; or the wandering albatross that ranges over the whole of the oceans south of the Equator. It will be found that individual species of birds, have, as a rule, a much wider distribution than individual species of mammals, which is attributable to their much greater mobility. The same applies to many species of fish and other marine animals, and for the same reason. The ocean, as a medium, gives them a greater range of space through which they may wander, and So there are many globe-encircling forms, such as the great rorquals or finner whales and various sharks and other fishes.
It would be possible to continue almost indefinitely the many. phases of this fascinating subject, but enough has here been said to give the reader some idea of the distribution of animal and plant life over the surface of the earth and how it comes about.
Text read from the printed page and cleaned; old type leaves the occasional error.