16 INTRODUCTION TO EVOLUTION 



body was no longer buoyed up by surrounding water (Fig. 2.1); (2) an 

 air bladder connected to the mouth cavity and capable of being used as 

 a simple lung for respiration in the air; (3) nostrils which opened from 

 the exterior into the mouth cavity and thus made possible the breathing 

 of air with the mouth closed or otherwise employed. (In most fishes the 

 nostrils connect to closed pouches containing the sense organs of smell 

 and do not open into the mouth cavity.) 



We may picture some of these crossopterygian fishes as making use of 

 their preadaptations to crawl from their fetid pools, probably at first in 

 search of fresher and less crowded ones (Romer, 1959). Presumably the 

 first overland excursions were brief. And probably very few of the 

 crossopterygian fishes succeeded in making even this much departure from 



Nostrils connected 

 Air bladder connected to mouth cavity, 



to pharynx. 



Lobe fins 



FIG. 2.1. Diagram of crossopterygian fish dissected to show three ways in which it 

 was preadapted for life on land. 



ancestral habits. Pioneering is seldom a mass phenomenon. Eventually, 

 however, there must have arisen small populations of crossopterygian de- 

 scendants increasingly emancipated from life in the water and finally 

 making use of it only as a place to spawn and lay their eggs, as most 

 amphibians do to this day. This increasing emancipation from life in 

 the water would be accomplished by that postadaptation of which we have 

 spoken. The principal mechanism involved in the change from water to 

 air was doubtless the positive aspect of natural selection — the favoring 

 of individuals possessing changes (arisen through mutation) which more 

 adequately fitted them for life on land than their fellows. 



"Many Are Called but Few Are Chosen" 



We have stressed the point that the numbers of individuals involved 

 in making the dramatic change from water to air were probably small. The 



