286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



two pairs of strong articulated limbs terminated by movable digits, 

 though in some cases, as in snakes, in other ways. 



It is generally agreed that animal life originated in water, whether 

 in marshes, estuaries, or the sea is unimportant. But what is impor- 

 tant is that the basic evolutionary pattern was aquatic and that this 

 pattern has been carried over with modifications onto the land, the 

 animals most perfectly adjusted to strictly terrestrial conditions being 

 vertebrates, arthropods, and mollusks (gastropods) , representatives of 

 the three dominant and most diversified groups of aquatic animals. 



Against this background vertebrate life must develop, hold its own, 

 or perish. Most important for the continued existence of animal types 

 through geological and other changes is their ability to produce new 

 forms capable of meeting new conditions — that is, their capacity for 

 evolution. The evolutionary history of any animal type is dependent 

 upon three coordinate factors : First, the inherent ability to produce 

 variants or mutants, either micromutants differing but little from the 

 parent stock, or macromutants differing more or less widely from the 

 parent stock ; second, the existence of an adequate physiological balance 

 in the mutants that are produced; and third, the ability of such mu- 

 tants as may appear to find a habitat where the food supply is 

 adequate, the physical and chemical surroundings are suitable, and 

 there is no insurmountable competition. 



In the course of constant geological and other changes new habitats 

 appear from time to time, and established habitats become unsuited 

 for the existing fauna. The ecological vacua thus created may be 

 filled by modifications of the local fauna, or by immigrants from out- 

 side that possess or develop adaptations suited to the new conditions. 

 Thus the large grazing and browsing mammals of the South American 

 Pliocene and Pleistocene have given way to a great number of highly 

 diversified rodents, and the ecological niche occupied by the great 

 marine reptiles of the past has been preempted by the present mam- 

 malian cetaceans. 



Each vertebrate class has the potentiality of developing new forms 

 capable of occupying a considerable range of habitats. Thus each 

 class includes a considerable number of peripheral or fringe forms 

 arranged irregularly about a generalized central type. The five 

 vertebrate classes, fishes, amphibians, reptiles, birds, and mammals, 

 including their peripheral types, may be briefly characterized as 

 follows : 



CHARACTERISTICS OF VERTEBRATE CLASSES 



FISHES 



The fishes were the first vertebrates to appear as fossils, in the 

 Silurian. Fishes are all aquatic, though a number, both fresh-water 



