42 ESSAY ON CLASSIFICATION. 



be expected of those, whose studies are chiefly devoted to 

 the investigation of the structure of our globe. 



To appreciate fully the importance of this question of 

 the gradation of animals, and to comprehend the whole 

 extent of the difficulties involved in it, a superficial ac- 

 quaintance with the perplexing question of the order of 

 succession of animals in past geological ages is by no 

 means sufficient. On the other hand, a complete familia- 

 rity with the many attempts which have been made to 

 establish a correspondence between the two, and with all 

 the crudities which have been published upon this sub- 

 ject, might dispel every hope to arrive at any satisfactory 

 result upon this subject, did it not now appear that the 

 inquiry, to be conducted upon its true ground, must be cir- 

 cumscribed within different limits. The results at which 

 I have already arrived, since I have perceived the mis- 

 take under which investigators have been labouring thus 

 far in this respect, satisfy me that the point of view, under 

 which I have presented the subject here, is the true one; 

 and that, in the end, the characteristic gradation exhibited 

 by the orders of each class will present the most striking 

 correspondence with the character of the succession of the 

 same groups in past ages, and afford another startling 

 proof of the admirable order and gradation in the de- 

 grees of complication of the structure of animals, which 

 have been established from the very beginning and main- 

 tained throughout all time. 



SECTION IX. 



RANGE OF THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



The surface of the earth being formed partly by water 

 and partly by land, and the organization of all living 



