Page:Popular Science Monthly Volume 77.djvu/86

80 fishes, and to such reptiles as the herbivorous dinosaurs of the Upper Cretaceous.

This brief consideration of the external features of adaptation leads us to glance at groups of animals. We here observe the influence of geographic distribution; we observe the adaptive radiation of groups both continental and local.

Continental Adaptive Radiation.—Among the Tertiary mammals we can actually trace the giving off of radii in several, sometimes in all, directions for the purpose of taking advantage of every opportunity to secure food, to escape enemies, and to reproduce kind, the three phenomena of the struggle for existence. Among such well-known quadrupeds as the horses, rhinoceroses and titanotheres the modifications involved in these radiations can be clearly traced. Thus the history of the life of continents presents a picture of contemporaneous radiations in different parts of the world. We observe the contemporaneous and largely independent radiations of the hoofed animals in South America, in Africa and in the great continent comprising Europe, Asia and North America.

Through the laws of parallelism and convergence each of these radiations produced a greater or less number of analogous groups.

While originally independent, the animals thus evolved separately as autochthonous types in many cases finally mingled together as migrant or invading types.

We may thus work out gradually the separate contributions of the great land masses of North America, South America, etc., to the mammalian fauna of the world. As a rule the greater the continents the more important and fundamental the orders or larger groups of mammals which have radiated in them; the lesser land masses and continental islands, like Australia, have been less favorable to wide adaptive radiation. One of the most interesting features of adaptive radiation is that it may also occur locally.

Local Adaptive Radiation.—On a smaller scale are the local adaptive radiations which occur through segregation of habit and local isolation in the same general geographic region wherever physiographic and climatic differences are sufficiently great to produce local differences in food supply or other local factors of change. This principle is well known among living animals, and it is now being demonstrated among many of the Tertiary mammals, remains of four or five distinct genetic series having been discovered in the same geologic deposits.

The existence of multiple phyla of related animals, as of the rhinoceroses, horses and titanotheres in the same localities is due partly to the operation of the law of local adaptive radiation.

This is conspicuously the case among the titanotheres, for example, the chief evolution of which can be traced in the Rocky Mountain