176 ZOOLOGICAL PHILOSOPHY 



most numerous, live only in the water, as I shall hereafter mention 

 (p. 246) ; that it is exclusively in water or very moist places that 

 nature achieved and still achieves in favourable conditions those direct 

 or spontaneous generations which bring into existence the most simply 

 organised animalcules, whence all other animals have sprung in turn. 



We know that the infusorians, polyps, and radiarians live exclusively 

 in the water ; and that some worms even live in it while the rest dwell 

 only in very moist places. 



Now the worms appear to form one initial branch of the animal 

 scale, and it is clear that the infusorians form the other branch. We 

 may suppose therefore that such worms as are completely aquatic 

 and do not live in the bodies of other animals, Gordius, for instance, and 

 many others that we are not yet acquainted with, have doubtless 

 become greatly diversified in the water ; and that among these aquatic 

 worms, those which afterwards became accustomed to exposure to 

 the air have probably produced the amphibian insects such as gnats, 

 mayflies, etc., etc., while these in turn have given existence to all the 

 insects which live altogether in the air. Several races of these again 

 have changed their habits as a result of their environment and con- 

 tracted a new habit of living hidden away in solitude : hence the origin 

 of the arachnids, nearly all of which live also in the air. 



Finally, those arachnids that frequented water, and gradually became 

 accustomed to live in it until at last they altogether ceased to live 

 in the air, led to the existence of all the crustaceans ; this is clearly 

 indicated by the aflanities which connect the centipedes with the 

 millipedes, the miUipedes with the woodlice, and these again v\ith 

 Asellus, shrimps, etc. 



The other aquatic worms, which are never exposed to the air, 

 would have developed in course of time into many different races with 

 a corresponding advance in the complexity of their organisation. They 

 would thus have led to the formation of the annelids, cirrhipedes and 

 molluscs, which form together an unbroken portion of the animal 

 scale. 



There seems to us to be a great hiatus between the known molluscs 

 and the fishes ; yet the molluscs whose origin I have just named 

 have led to the existence of the fishes through the medium of other 

 molluscs that have yet to be discovered, and it is manifest that the 

 fishes again have given rise to the reptiles. 



As we continue to examine the probable origin of the various animals, 

 we cannot doubt that the reptiles, by means of two distinct branches, 

 caused by the environment, have given rise, on the one hand, to the 

 formation of birds and, on the other hand, to the amphibian mammals, 

 which have in their turn given rise to all the other mammals. 



