EVOLUTION OF THE AMPHIBIA 433 



stages which breathed by gills required then, as they do now, a 

 watery medium. Next, fertilisation was external, and for the 

 sperms to be able to find the eggs, there must be a liquid 

 medium for them to swim in. Lastly, amphibia breathe largely 

 through their skins, and these must be moist to enable the 

 gaseous exchange to take place. 



The transition from water to air necessitated a development 

 of the olfactory organs to greater sensitiveness, for the concen- 

 tration of substances in water is very much greater than that 

 which can be obtained in air. The result was an increase in 

 development of the olfactory organs and of the corresponding 

 centres in the forebrain. The latter development accompanied 

 and perhaps assisted the formation of the cerebral hemispheres, 

 which are regarded as connected with an adaptation to the 

 poor oxygen- content of the water in which the amphibia and 

 their ancestors evolved. 



It appears, therefore, that the transition from aquatic to 

 terrestrial life was accomplished without any very striking 

 changes or modification of organs, but it must be remembered 

 that the function of these organs is controlled by the pattern 

 of nerve-fibres in the central nervous system, and it becomes 

 necessary to inquire whether the transition necessitated any 

 great neurological rearrangement. Two aspects of the transi- 

 tion will be considered, regarding breathing and locomotion. 

 In connexion with respiration, it will be remembered that the 

 amphibia breathe by means of respiratory movements per- 

 formed by the visceral muscles in the floor of the mouth, in a 

 manner very similar to that of the fish . The only real difference 

 is that whereas the fish take in water and pass it back and out 

 through the gill-slits, the amphibia take in air and pass it 

 back and into the lungs. The mechanism is the same, and it 

 is obvious that the transition from water to air involved no 

 functional rearrangement of importance as regards respiration. 



The same holds true with regard to locomotion. The 

 amphibia were clumsy sluggish beasts with bodies dispropor- 

 tionately large in comparison with their limbs. As a conse- 

 quence, the body was not supported by the limbs but its ventral 

 surface dragged along the ground. The limbs stuck out at 



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