FUNDAMENTAL RELATIONS OF ANIMALS 97 



tends frequently only to the genera or the species. But whatever 

 be the extent of their range in time, we shall see presently that all 

 these types bear, as far as the order of their succession is concerned, 

 the closest relation to the relative rank of living animals of the same 

 types compared with one another, to the phrases of the embryonic 

 growth of these types in the present day, and even to their geo- 

 graphical distribution upon the present surface of our globe. I will, 

 however, select a few examples for further discussion. Among Echi- 

 noderms the Crinoids are for a long succession of periods the only 

 representatives of that class; next follow the Starfish, and next the 

 Sea-Urchins, the oldest of which belong to the type of Cidaris and 

 Echinus, followed by Clypeastroids and Spatangoids. No satisfactory 

 evidence of the existence of Holothurias has yet been found. Among 

 Crustacea, while Trilobites are the only Crustacea of the oldest 

 palaeozoic rocks, there is found in the Jurassic period a carcinological 

 fauna entirely composed of Macrura, to which Brachyura are added 

 in the tertiary period. The formations intermediate between the 

 older palaeozoic rocks and the Jura contain the remains of other 

 Entomostraca, and later of some Macroura also. In both classes the 

 succession of their representatives in different periods agrees with 

 their respective standing, as determined by the gradation of their 

 structure. 



Among plants we find in the Carboniferous period prominently 

 Ferns and Lycopodiaceas; in the Triassic period Equisetaceae and 

 Conifers prevail; in the Jurassic deposits, Cycadeae, and Monocotyle- 

 doneas; while later only Dicotyledoneae take the lead. The icono- 

 graphic illustration of the vegetation of past ages has of late ad- 

 vanced beyond the attempts to represent the characteristic features 

 of the animal world in different geological periods. 



Without attempting here to characterize this order of succession, 

 this much follows already from the facts mentioned, that while the 

 material world is ever the same through all ages in all its combina- 

 tions, as far back as direct investigations can trace its existence, or- 

 ganized beings, on the contrary, transform these same materials into 

 ever new forms and new combinations. The carbonate of lime of all 

 ages is the same carbonate of lime in form as "vvell as composition, as 

 long as it is under the action of physical agents only. Let life be in- 

 troduced upon earth, and a Polyp builds its coral out of it, and each 



