PRINCIPLES OF ZOOLOGY. 33 



25. Nor are our researches to be limited to the animals 

 now living. There are buried in the crust of the earth the 

 remains of a great number of animals belonging to species 

 which do not exist at the present day. Many of these 

 remains present forms so extraordinary that it is almost im- 

 possible to trace their alliance with any animal now 

 living. In general, they bear a striking analog}' to the em- 

 bryonic forms of existing species. For example, the curi- 

 ous fossils known under the name of Trilobites (Fig. 156) 

 have a shape so singular that it might well be doubted to 

 what group of articulated animals they belong. But if we 

 compare them with the embryo crab, we find so remarkable 

 a resemblance that we do not hesitate to refer them to the 

 crustaceans. We shall also see that some of the Fishes 

 of ancient epochs present shapes altogether peculiar to them- 

 selves, (Fig. 157,) but resembling, in a striking manner, the 

 embryonic forms of our common fishes. A determination 

 of the successive appearance of animals in the order of time 

 is, therefore, of much importance in assisting to decide the 

 relative rank of animals. 



26. Besides the distinctions to be derived from the varied 

 structure of organs, there are others less subject to rigid 

 analysis, but no less decisive, to be drawn from the imma- 

 terial principle with which every animal is endowed. It is 

 this which determines the constancy of species from genera- 

 tion to generation, and which is the source of all the varied 

 exhibitions of instinct and intelligence which we see dis- 

 played, from the simple impulse to receive the food which is 

 brought within their reach, as observed in the polyps, through 

 the higher manifestations, in the cunning fox, the sagacious 

 elephant, the faithful dog, to the exalted intellect of man. 

 which is capable of indefinite expansion. 



27. Such are some of the general aspects in which we 

 are to contemplate the animal creation. Two points of 



