OKIGIN OF THE CEREBRAL HEMISPHERES 445 



Now these forms are not the most successful types of fishes. 

 From the standpoint of biological efficiency they are surpassed 

 by the elasmobranchs which from early Silurian times have 

 dominated the salt waters and by the teleosts which dominate 

 the fresh waters and now compete with the sharks for the mastery 

 of the oceans. The surviving members of these archaic types 

 exist to-day not by reason of the functional superiority of their 

 hollow cerebral hemispheres over the massive soUd thickenings 

 of the primitive endbrain seen in teleosts, but rather because 

 they have, as it were, hidden themselves away in crannies of 

 the environment not sought after by the more enterprising groups 

 of fishes. And here perhaps is the solution of our problem. 



Fishes with evaginated cerebral hemispheres are inhabitants 

 of sluggish waters; they and their allies, the primitive ganoids, 

 are mudfishes. Lull ('18) has called attention to the fact that 

 this type of fishes came into prominence at a geological period 

 (the late Silurian and early Devonian) when extensive continental 

 fresh- water lakes and streams were drying up. The fish fauna 

 of these waters was faced with the alternative of gradual modi- 

 fication in adaptation to seasonal drought or extinction. The 

 more highly speciaUzed species inhabiting these stagnant waters 

 undoubtedly perished, as their 'senile' type of organization was 

 unable to make the necessary readjustment; but the more 

 generahzed forms, whose undifferentiated tissues retained the 

 plasticity and adaptabihty of the 'young' type, met the emer- 

 gency by supplementing their water-breathing apparatus by 

 air-breathing organs of various sorts. Thus arose the lungfishes 

 and the Amphibia. 



Most of the profound modifications of the vertebrate body 

 necessitated by the transfer from the aquatic habitat to land 

 can readily be understood. The adaptive valile of lungs, limbs, 

 etc., is obvious; and, given variations of appropriate range in a 

 plastic type, the metamorphosis may be conceived as carried 

 through in accordance with the principle of survival of those 

 variations most fit to meet the changed conditions. But the 

 reason for the association of evaginated cerebral hemispheres 

 with the other and obviously adaptive characters related with 



