76 THE DOCTRINE OF DESCENT. 



served in the Turrilites, Scaphites, Baculites, and various 

 others. The prime of the great sea-lizards is also past, but 

 the marshes of the Wealden period harboured new forms 

 of colossal land-lizards. The long-tailed cray-fishes are 

 joined by the true crabs, the most highly developed 

 forms of the class. In the Oolite and Chalk also occur 

 the chief of the sea-urchin-like Echinodermata. As yet 

 we have not mentioned the class of Echinodermata, in 

 order that we might here point out in conjunction several 

 of the more important phases of their geological occur- 

 rence. Desor,* a distinguished judge of this class, has 

 lately examined how in this large group of Echinidae 

 the progress of organization is gradually manifested, on 

 which occasion he was induced to make some general 

 reflections on the principle of progressive development, 

 as applied to the Echinodermata, probably known to all 

 our readers in their representatives the star-fish and sea- 

 urchins. If articulate, as well as vertebrate, animals attain 

 a higher grade of development by the differentiation of the 

 consecutive segments of the body, the superior unity, and 

 therewith higher perfection, of the Echinoderm's body is 

 evinced when the rays, or so-called antimera, give way to 

 the unity of the whole. 



The more distinct these elements are, that is to say, the 

 more independent they remain, the lower is, not only the 

 articulate animal, but also the Echinoderm. Accord- 

 ingly, the star-fish, and to some extent the feather-stars, 

 stone -lilies, or crinoids, occupy the lowest rank. But 

 here, unluckily, palaeontological evidence again aban- 

 dons us. Only so much is certain, that in the older 

 fossiliferous strata both divisions are abundantly repre- 



* Eu'letin de la Societe des Sciences Naturelles de Neufchatel, IX. 2. 



