DISTRIBUTION OF ANIMALS. 511 



spatangi and holothuridse. Is this not quite 

 a parallel case with the monkeys and pachy- 

 derms ? for once crinoids were the only rep- 

 resentatives of the class of echinoderms. May 

 we not say the same of crocodiles when com- 

 pared with the ancient gigantic saurians ? or 

 are the crocodiles, as an order, distinct from 

 the other saurians, and really higher than the 

 turtles ? Innumerable questions of this kind, 

 of great importance for zoology, are sug- 

 gested at every step, as soon as we compare 

 the present distribution of animals with that 

 of the inhabitants of former geological pe- 

 riods. Among Crustacea, it is very remark- 

 able that trilobites and limulus-like forms 

 are the only representatives of the class dur- 

 ing the paleozoic ages ; that macrourans pre- 

 vailed in the same manner during the second- 

 ary period ; and that brachyurans make their 

 appearance only in the tertiary period. Do 

 you discover in your results any connection 

 between such facts and the present distribu- 

 tion of Crustacea? There is certainly one 

 feature in their classification which must ap- 

 pear very striking, that, taken on a large 

 scale, the organic rank of these animals agrees 

 in the main with their order of succession in 

 geological times ; and this fact is of no small 



