THE CONTINUITY OF LIFE 13 



reference to the problem in hand and with no regard to the 

 future inconveniences which may arise from a certain form 

 or arrangement. 



To learn this histpry we must turn to the comparative 

 anatomy of vertebrates. Some of them are still so similar 

 to the early stages of our own development that we may 

 almost look upon them as our former selves ; others represent 

 development along other lines to which their environment and 

 its necessities have brought them, and they show us what we 

 might have been, had chance led us in their direction. 



The first period of vertebrate history was an aquatic one, 

 in which the environment was represented, not merely by the 

 water, which developed a certain kind of respiration, and al- 

 lowed a style of locomotive organs inadmissible on land, but 

 by the vast hordes of carnivorous enemies generated in the 

 depths of the ocean; yet, through these struggles was gained 

 an exoskeletal armor with which to ward off the attacks of 

 the powerful molluscs and crustaceans of the Silurian seas; 

 and of the armor plates thus obtained the relics are still re- 

 tained in the cranial region, forming the dermal bones of the 

 skull (frontals, parietals, squamosals, etc.). 



Profound changes became necessary when our ancestors 

 left the ocean and sought refuge in the marshes and upon 

 land; changes not merely in the mode of respiration, but in 

 the entire skeletal and muscular system, owing to the great 

 difference in specific gravity between water and air. Differ- 

 ences in food caused modifications in the digestive system, 

 and all surfaces exposed to the air developed glands in pro- 

 fusion to resist the drying effect of sun and wind. During 

 this period were acquired pentadactylous extremities, lungs 

 and larynx, and the salivary and lacrimal glands. The or- 

 ganism became modified in countless ways, as the attempts to 

 inhabit dry lands, apart from the marshes, ushered in the next 

 great period, that of the rocks and plains. 



Here began a complete aerial respiration, the development 

 of the permanent kidneys, which replaced the Wolffian bodies 

 of amphibians, and the formation of a cornified epidermis, with 



