96 GEADATION AMONG ANIMALS. 



with the others at the beginning, such as Insects 

 among Articulates, or Reptiles, Birds, and Mam- 

 malia among Vertebrates, are always introduced 

 at the time when the conditions essential to their 

 existence are established, as, for instance, Rep- 

 tiles, at the period when the earth was not fully 

 redeemed from the waste of waters, and exten- 

 sive marshes afforded means for the half-aquatic, 

 half-terrestrial life even now characteristic of all 

 our larger Reptiles, while Insects, so dependent 

 on vegetable growth, make their appearance with 

 the first forests ; so that we need not infer, be- 

 cause these and other classes come in after the 

 earlier ones, that they are therefore a growth out 

 of them, since it is altogether probable that they 

 would not be created till the conditions necessary 

 for their maintenance on earth were established. 

 From a merely speculative point of view it 

 seems to me natural to suppose that the physical 

 and the organic world have progressed together, 

 and that there is a direct relation between the 

 successive creations and the condition of the 

 earth at the time of those creations. We know 

 that all the beings of the Silurian and Devonian 

 periods were marine ; the land, so far as it existed 

 in their time, consisted of great beaches, and along 

 those shores, wherever any part of the continent 

 was lifted above the level of the waters, the Silu- 

 rian and Devonian animals lived. Later in the 



