26 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



invertebrate faunas of the Niagara limestone, the saurians of the Jurassic 

 and Cretaceous, or the very similar mammalian faunae^ of certain stages of 

 the Tertiary, 



(/) Radiation and Depression of Life. 



Circumstances of variable kinds have determined the abundance or 

 paucity of life at irregular intervals of the world history. By radiation is 

 understood the increase not only in number of individuals, but of varieties, 

 species, genera, and even large groups, reaching out in all directions to find 

 unoccupied niches in the scheme of existence where food or protection, 

 breeding-places or homes, might be enjoyed with the minimum of loss, 

 Osborn has called this, very aptly, "adaptive radiation," ^ 



Radiations are of two kinds. The greater radiations of the Classes of 

 animal life, where each successively — fish, amphibian, reptile, and mammal — 

 asserted the dominance conferred by superior endowments in organization 

 and for a time reigned as masters of the world. These radiations, which have 

 given rise to such terms as age of fishes, age of amphibians, age of reptiles, 

 age of mammals, were the result of the operation of the law of continuous 

 improvement in life and has a broad but important bearing on the paleogeo- 

 graphic problem as it was during the periods of expansion of each group 

 that the closest response to the environment was developed. 



Lesser radiations were governed by more evident factors and the dis- 

 covery of a unit containing an unusual number of individuals, varieties, or 

 species of any smaller group of life should direct inquiry into the cause, 

 and this may well be the key to the geographic conditions of the time. 

 Such radiations may follow the entrance of a migrant fauna into a new 

 region where enemies do not exist or are not in sufficient numbers to restrain 

 the natural increase. Classic examples from our own experience are the 

 remarkable increase of the English sparrow, the cotton-boll weevil, or the San 

 Jose scale in America, or the rabbits in Australia, Climatic or surface changes, 

 followed by the inevitable alteration of the vegetation, might give the ad- 

 vantage to a small or large group and start it upon a career of supremacy.^ 



Depressions of life are the exact correlatives of the radiations. Untoward 

 physical conditions, such as the increasing salinity of the remnant seas of 

 late Silurian time or the unknown conditions which caused the decrease of 

 the Pelmatozoa in the Permian, illustrate this point. One has but to think 

 of the effect of great droughts or blizzards on the plains of Argentina, 

 Patagonia, North America, and Australia to realize what severe climatic 

 changes may do. The introduction of enemies, as the trypanosomes of the 

 sleeping sickness in Africa, or the immigration of dominating forms, may 



^ Osborn, H. F., Age of Mammals, p. 96 and following. 



2 Osborn, H. F., The Law of Adaptive Radiation, Amer. Nat., vol. 36, p. 353, 1902, and 



Age of Mammals, p. 22. 



3 Osborn, in the article cited above, gives examples of adaptive radiations in the food habits 



and mode of locomotion. 



