THE CHAIN OF LIFE TRACED BACKWARD 9 



the leading groups of marine invertebrates run with- 

 out interruption back to the Lower Cambrian, and 

 some of them still farther. Thus it would appear 

 that for long ages before the introduction of land or 

 air-breathing animals of any kind, the sea swarmed 

 with animal life, which was almost as varied as that 

 which now inhabits it. The reasons of this would 

 seem to be that the better support given by the 

 water makes less demands upon organs for me- 

 chanical strength, that the water preserves a more 

 uniform temperature than the air, and that arrange- 

 ments for respiration in water are less elaborate 

 than those necessary in air. Hence the conditions 

 of life are, so to speak, easier in water than in air, 

 more especially for creatures of simple structure and 

 low vital energy. Besides this, the waters occupy 

 two-thirds of the surface of the earth, and in earlier 

 periods probably covered a still greater area. 



We are now in a position to understand that the 

 Animal Kingdom had not one but many beginnings, 

 its leading types arriving in succession throughout 

 geological time. Thus the special beginning of any 

 one line of life, or those of different lines, might 

 form special subjects of inquiry ; but our present 

 object is to inquire as to the first or earliest in- 



