148 EVOLUTION AND GENETICS 



to be taken anywhere in the development of the higher classes. 

 The transition from aquatic to terrestrial life between fishes and 

 amphibia, or better still between fishes and reptiles since the am- 

 phibia are still in a transitional state, involves fundamental 

 changes of no slight degree. 



Water as an environmental medium contrasts with the air in 

 several important particulars. (1) Because of its higher specific 

 gravity it buoys up the organism with greater force. (2) It pre- 

 vents the loss of water from the body by evaporation. (3) It 

 offers greater resistance to motion on the part of immersed bodies. 

 (4) It presents different conditions of visibility. 



The Demands of Terrestrial Life. While removal from the 

 water to the air demands relatively slight modifications in response 

 to some conditions, to others extreme adjustments are necessary. 

 The body is no longer buoyed up completely by the surrounding 

 medium, but is of so much higher specific gravity that it must 

 rest on the ground. This demands a different type of locomotion 

 involving even in primitive forms limited points of support for the 

 body, and consequently a more rigid skeletal structure. The in- 

 tegument must be modified to conserve moisture within the body 

 in proportion to the dryness of the air. While aquatic animals need 

 streamline bodies if they are to move rapidly, in the air the 

 resistance is so much less that body form is of relatively little im- 

 portance. The difference of visibility in the air makes possible 

 some modifications of the eye. In addition, the two media demand 

 entirely different respiratory organs, and correspondingly different 

 circulation. 



For complete separation from the water as a habitat, the de- 

 velopment of fetal membranes is an apparent essential, due no 

 doubt to the delicacy of eml)ryonic tissue and the resulting neces- 

 sity for protection against dessication and mechanical injury. 



Transitional Forms. A first step in tracing the theoretical 

 portion of vertebrate evolution is to decide what characters would 

 be present in a species capable of developing into terrestrial forms. 

 To substantiate the hypothetical ancestor, it is necessary to de- 

 termine whether or not such organisms have ever existed. Finally 

 we must determine whether environmental conditions during the 

 geological period of the transition were favorable for the change. 



Since the earliest vertebrates were fishes, the ancestors of the 

 terrestrial forms must have belonged to this class. In order to 



