96 ESSAY ON CLASSIFICATION 



they exhibit in successive formations/^^ what is their geographical 

 distribution, only those can fully appreciate, who have had a hand 

 in the work. And even now, how many important questions still 

 await an answer! 



One result, however, now stands unquestioned: the existence 

 during each great geological era of an assemblage of animals and 

 plants differing essentially for each period. And by period I mean 

 those minor subdivisions in the successive sets of beds of rocks which 

 constitute the stratified crust of our globe, the number of which 

 is daily increasing, as our investigations become more extensive and 

 more precise. ^-^ What remains to be done is to ascertain with more 

 and more precision the true affinities of these remains to the animals 

 and plants now living, the relations of those of the same period to 

 one another and to those of the preceding and following epochs, the 

 precise limits of these great eras in the development of life, the 

 character of the successive changes the animal kingdom has under- 

 gone, the special order of succession of the representatives of each 

 class, their combinations into distinct faunas during each period, 

 not to speak of the causes or even the circumstances under which 

 these changes may have taken place. 



In order to be able to compare the order of succession of the 

 animals of past ages with some other prominent traits of the animal 

 kingdom, it is necessary for me to make a few more remarks upon 

 this topic. I can, fortunately, be very brief, as we possess a text-book 

 of Palaeontology arranged in zoological order, in which every one 

 may at a glance see how, throughout all the classes of the animal 

 kingdom, the different representatives of each in past ages are dis- 

 tributed in the successive geological formations. ^-^ From such a cur- 

 sory survey it must appear that while certain types prevail during 

 some periods, they are entirely foreign to others. This limitation is 

 conspicuous with reference to entire classes among Vertebrata, Avhile 

 in other types it relates more to the orders or to the families and ex- 



^'See note 33, above. 



^^ At first only three great periods were distinguished, the primary, the secondary, 

 and the tertiary; afterwards, six or seven (Henry Thomas de la B^che); later, from ten 

 to twelve; now, the number is almost indefinite, at least undetermined in the present 

 stage of our knowledge, when many geologists woidd only consider as subdivisions of 

 longer periods what some paleontologists are inclined to consider as distinct periods. 



^-^ I allude to the classical work of Francois J. Pictet, Traits elementaire de Paleonto- 

 logie (4 vols., Paris, 1844-1845). 



