LAMARCK'S THEORY OF DESCENT 



341 



still unknown — there are doubtless a great many dif- 

 ferent aquatic forms ; and that among these aquatic 

 worms, those which afterwards habitually expose 

 themselves to the air have probably produced am- 

 phibious insects, such as the mosquitoes, the ephem- 

 eras, etc., etc., which have successively given origin 

 to all the insects which live solely in the air. But 

 several races of these having changed their habits by 

 the force of circumstances, and having formed habits 

 of a life solitary, retired, or hidden, have given rise to 

 the arachnides, almost all of which also live in the 

 air. 



" Finally, those of the arachnides which have fre- 

 quented the water, which have consequently become 

 progressively habituated to live in it, and which finally 

 cease to expose themselves to the air — this indicates 

 the relations which, connecting the Scolopendrse to 

 Julus, this to the Oniscus, and the last to Asellus, 

 shrimps, etc., have caused the existence of all the 

 Crustacea. 



" The other aquatic worms which are never exposed 

 to the air, multiplying and diversifying their races 

 with time, and gradually making progress in the 

 complication of their structure, have caused the 

 formation of the Annelida, Cirripedia, and molluscs, 

 which together form an uninterrupted portion of the 

 animal scale. 



" In spite of the considerable hiatus which we ob- 

 serve between the known molluscs and the fishes, the 

 molluscs, whose origin I have just indicated, have, by 

 the intermediation of those yet remaining unknown, 

 given origin to the fishes, as it is evident^that the 

 latter have given rise to the reptiles. 



" In continuing to consult the probabilities on the 

 origin of different animals, we cannot doubt but that 

 the reptiles, by two distinct branches which circum- 

 stances have brought about, have given rise on one 

 side to the formation of birds, and on the other to 



