PRINCIPLES OF ZOOLOGY. 7 



23. Not less striking are the relations that exist between 

 animals and the regions they inhabit. Every animal has its 

 home. Animals of the cold regions are not the same as those 

 of temperate climates ; and these latter, in their turn, differ 

 from those of tropical regions. Certainly, no one will main- 

 tain it to be the effect of accident that the monkeys, the most 

 perfect of all brute animals, are found only in hot countries ; 

 or that it is by chance that the white bear and reindeer in- 

 habit only cold regions. 



24. Nor is it by chance that the largest of all animals, of 

 every class, as the whales, the aquatic birds, and the sea- 

 turtles, dwell in the water rather than on the land ; and while 

 this element affords freedom of motion to the largest, so is it 

 also the home of the smallest of living things. 



25. In the study of zoology we must not confine our re- 

 searches to animals now in existence. 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 pre- 

 sent day ; many of these remains present forms so extraor- 

 dinary, that it is almost impossible to trace their connection 

 with any animals now living. In general, they bear a striking 

 analogy to the embryonic forms of existing species ; for ex- 

 ample, the curious fossils known under the name of Tri- 

 lobites (Fig. 378) 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 hesitate not to refer them 

 to the crustaceans. We shall also see that some of the fishes 

 of ancient epochs present shapes entirely peculiar to them- 

 selves (Fig. 3/9), resembling in a striking manner the em- 

 bryonic forms of some of our common fishes. A determina- 

 tion of the successive appearance of animals, in the order of 

 time, is therefore of much importance in assisting us to deter- 

 mine their relative zoological rank. 



26. Besides the distinctions derived from the varied struc- 

 ture of organs, there is another less subject to rigid analysis, 

 but no less decisive, to be drawn from the immaterial principle, 

 with which every animal is endowed. It is this vital principle 

 which determines the constancy of species, from generation to 

 generation, and which is the source of all the varied exhibi- 

 tions of instinct and intelligence which we see displayed, from 



