68 RAMBLES OF A NATURALIST. 



in respect to the Vertebrata if Keptiles, Birds, and 

 Mammals are wanting in these primitive faunas a 

 simple explanation of their absence would be fur- 

 nished on the supposition of the external conditions 

 being incompatible with their mode of life. 



Moreover, in proportion as we rise to the more 

 modern geological strata, we find these types modi- 

 fied ; in some cases making a gradual advance towards 

 perfection, up to the appearance of Man upon the 

 globe, much in the same manner as we observed the 

 progressive advancement of the young Terebella as 

 it passed through each successive phase of its ex- 

 istence ; while at another time the most perfect 

 species seem lost, remaining to us only in their in- 

 ferior forms, and thus giving rise to recurrent series, 

 as we still see at the present day in certain animals, 

 as, for instance, in the Lernea, which seems to re- 

 trograde by the very progress of its development.* 



* The Lerneas are Crustaceans, which are extremely curious, on 

 account of the metamorphoses which they undergo, the result of 

 which, more especially in the females, is to induce a progressive 

 degradation. These animals in their early age bear a considerable de- 

 gree of resemblance to the Cyclops, a small Crustacean which occurs 

 in great abundance in our fresh waters. They are at this period 

 provided with a well-characterised frontal eye, whilst their natatory 

 legs enable them to swim freely ; but after having undergone several 

 moults, they finally affix themselves upon another animal, at whose 

 expense they subsist for the rest of their lives. The females attach 

 themselves by some means to the body of this animal, whilst the 

 males have their bodies attached to those of the females. The eyes 

 and the limbs become gradually atrophied and disappear, while the 

 different portions of the mouth become converted into a simple 

 sucker. The changes experienced by the males are less marked, but 

 the females become so completely degraded, that their nature was 



