94 SEA AND LAND 



variation, the stru<^gle for existence will brine;' about a stead- 

 fast gain in the fitness of a form for the duties of life in its 

 proper station, and that in time almost any measure of ad- 

 vance may be attained. But the failure of the marine forms to 

 \\\\\ their way to the organic and intellectual successes of the 

 hit^her life is suf^cient evidence that time and struijorle. infinite 

 toil and pain, ceaseless life and death, will not alone enable 

 species to win the upward way. Many other conditions, 

 which, in a question-begging manner, we terni the influences 

 of environment, must ent(;r into the inconceivably complicated 

 equation which determines the fate of living beings. The 

 struggle for existence has been as bitter in the seas as upon 

 the land, it might well be maintained that it is far more intense 

 in the water than in the animal realm, and it has endured for 

 a greater time. What, then, is the reason for the lagging 

 behind of the marine creatures ? 



It may well be that this slow advance, or rather, we should 

 term it, this wide-spread failure of the aquatic life in all that 

 regards elevation in structure and function, is due to many 

 different causes ; but there is one cause which may, of itself, 

 perhaps, in large part, account for the tardy evolution of the 

 inhabitants of the sea. This is the imperfect nature of the 

 breathing process which is inevitable in all truly aquatic 

 animals. Marine animals necessarily depend for their process 

 of respiration on the small amount of air which is dissolved 

 in the water, a part of which they appropriate by means of 

 their gills. The result is that in proportion to their size a 

 gill-bearing animal, at best, has perhaj^s not the tenth part 

 of the access to oxygen which is enjoyed by the ordinary 

 land forms. Now, on the amount of this gas which comes in 

 contact with the blood depends the share of nervous energy 



