16 VERTEBRATE ZOOLOGY 



place we shall concern ourselves with a study of the laws of adapta- 

 tion, following closely the analysis given by Osborn. 



THE LAWS OF ADAPTATION 



"The form evolution of the back-boned animals, beginning 

 with the pro-fishes of Cambrian and pre-Cambrian time, extends 

 over a period estimated at not less than 30,000,000 years. The 

 supremely adaptable vertebrate, body type begins to dominate 

 the living world, overcoming one mechanical difficulty after an- 

 other, and passes through the habitat zones of water, land, and 

 air. Adaptations in the motions necessary for the capture, stor- 

 age, and release of plant and animal energy continues to control 

 the form of the body and its appendages, but simultaneously the 

 organsim through mechanical and chemical means protects it- 

 self either offensively or defensively and also adapts itself to re- 

 produce and protect its kind." Osborn finds that there are two 

 great laws of adaptation: 



1. The law of convergence or parallelism of form in locomotor, offen- 

 sive, and defensive adaptations. There is a widespread tendency for 

 members of totally unrelated groups to develop similar body form 

 and similar external characters in adaptation to similar habitat zones. 

 Several adaptive types are readily distinguished and are herewith 

 listed : 



Aquatic or swimming animals. Among the thoroughly marine 

 types of vertebrates, whether they be fish, reptile or mammal, there 

 is in general the same fish-like body form, with the same sorts of 

 locomotor structures, paired and median fins. Apart from their 

 general aquatic adaptations the shark, ichthyosaur (reptile), and 

 dolphin (mammal) are profoundly different (Fig. 4). 



"The three mechanical problems of existence in the water hab- 

 itat are: first, overcoming the buoyancy of water either by 

 weighting down or increasing the gravity of the body or by the 

 development of special gravitating organs, which enable animals 

 to rise and descend in this medium; second, the mechanical prob- 

 lem of overcoming the resistance of water in rapid motion, which 

 is accomplished by means of warped surfaces and well-designed 

 entrant and re-entrant angles of the body similar to the ' stream- 

 lines ' of the fastest modern yachts; third, the problem of propul- 

 sion of the body, which is accomplished, first, by sinuous motion 



