•jt THE DOCTRINE OF DESCENT. 



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 Echinidse 

 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 at- 

 tain a higher grade of development by the differentiation 

 of the consecutive segments of the body, the superior 

 unity, and therewith higher perfection, of the Echino- 

 derm's body is evinced when the rays, or so-called an- 

 timera, 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, palseontological evidence again aban- 

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

 siliferous strata both divisions are abundantly repre- 

 sented. A highly remarkable and important intermediate 

 form is also known, found in the upper Silurian strata 

 of Dudley (Eucladia Johnsoni), the more important as 

 but few transitional forms between one order and another 

 * Bulletin de la Society des Sciences Naturelles de Neufchatel, IX. 2. 



