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 Scolopendre 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 
