Il6 THE EVOLUTION OF MAN. 



continued exclusively adapted to aquatic habits of life. It 

 was not till later that they adopted a land life. The earliest 

 fossils of terrestrial animals occur in the Devonian strata, 

 which were deposited in the beginning of the second great 

 division of the earth's history (the Palaeozoic Epoch). They 

 increase greatly in number in the deposits of the Coal and 

 Permian Periods. Even in these early formations many 

 terrestrial and air-breathing species, both of the Arthro- 

 pod and of the Vertebrate tribe, occur ; while their aquatic 

 ancestors of the Silurian Period breathed nothing but 

 water. This physiologically significant modification of the 

 mode of respiration is the most influential change that 

 affected the animal organism in the transition from water 

 to dry land. In the first place it caused the development 

 of an air-breathing organ, the lung, the water-breathing 

 gills having previously acted as respiratory organs. Simul- 

 taneously, however, it effected a remarkable change in the 

 circulation of the blood and in the organs connected with 

 this ; for these are always most closely correlated with the 

 respiratory organs. In addition to these, other organs also, 

 either in consequence of more remote correlation with the 

 respiratory organs, or in consequence of new adaptations, 

 were more or less modified. 



Within the Vertebrate tribe it was undoubtedly a branch 

 of the Primitive Fishes (Selachii) which, during the De- 

 vonian Period, made the first successful effort to accustom 

 itself to terrestrial life and to breathe atmospheric air. In 

 this the swimming-bladder was especially of service, for it 

 succeeded in adapting itself to respiration of air, and so 

 became a lung. The immediate consequence of this was 

 the modification of the heart and nose. While true Fishes 



