Doubtless, local oscillations in coast lines have taken place and are even in operation at present, but the time has passed when a dance of continents can be invoked to explain anomalies in animal distribution. Most of these will be found to have simple causes, when we know enough of the facts in the case to justify a hypothesis.
The laws governing animal distribution are reducible to three very simple propositions:
Every species of animal is found in every part of the earth having conditions fit for its existence, unless
(a) Its individuals have been unable to reach the region in question through barriers of some sort, or,
(b) Having, reached the region, the species is unable to maintain itself through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment, or else,
(c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the parent type.
In general, the different types of fishes are most specialized along equatorial shores. The processes of change through natural selection take place most rapidly there and produce more far-reaching modifications. The coral reefs of the tropics are the centers of fish-life, corresponding in fish economy to the cities in human affairs. The fresh water, the Arctic waters, the deep sea and the open sea represent ichthyic backwoods—regions where change goes on more slowly and in which archaic types survive.
The study in detail of the distribution of the fishes of the tropics, is most instructive. The study of the origin of the fish groups of Japan affords a fascinating introduction to its multifarious problems.