DEGRADATION OF ORGANISATION 85 



molluscs, which is quite pecuHar, exhibits no alternate swelling and 

 shrinking, never has a trachea or bronchi and in which the respired 

 fluid never enters by the animal's mouth. 



A respiratory cavity which has neither trachea nor bronchi nor 

 alternate swelling and shrinking, and in which the respired fluid 

 does not enter by the mouth, and which is adapted either for air or 

 water, cannot be a lung. To confuse such different things by the 

 same name is not to advance science but to retard it. 



The lung is the only respiratory organ that can give the animal 

 the faculty of having a voice. After the reptiles no animal has a 

 lung ; nor therefore a voice. 



I conclude that it is not true that there are molluscs which breathe 

 by limgs. If some in nature breathe air, so also do certain crustaceans 

 and all insects ; but none of these animals has true lungs, unless the 

 same name is to be given to very different objects. 



The molluscs also furnish proof of the progressive degradation that 

 we are investigating in the animal chain ; for their general organisation 

 is less perfect than that of fishes. But it is not so easy to recognise 

 the same degradation among the molluscs themselves ; for it is difficult 

 to distinguish in so numerous and varied a class what is due to the 

 degradation in question, from what is caused by the environment 

 and habits of these animals. 



The only two orders into which the large class of molluscs is 

 divided, are strongly contrasted by the importance of their dis- 

 tinctive characters. The animals of the first of these orders (cephalic 

 molluscs) have a very distinct head, eyes, jaws or a proboscis and 

 reproduce by copulation. 



All the moUuscs of the second order (acephaKc molluscs) on the con- 

 trary are destitute of a head, eyes, jaws, proboscis ; and they never 

 copulate for the purpose of reproduction. 



Now it can hardly be denied that the second order of molluscs 

 is inferior to the first as regards perfection of organisation. 



It is important, however, to remember that the absence of head, 

 eyes, etc., in the acephahc moUuscs is not wholly due to the general 

 degradation of organisation, since we find again at lower stages of 

 the animal chain, animals which have a head, eyes, etc. We have here 

 again apparently one of those deviations in the progress of perfection 

 of organisation that are produced by environment, and consequently 

 by causes foreign to those which make for a gradual increase of com- 

 plexity in animal organisation. 



When we come to consider the influence of the use of organs and of 

 an absolute and permanent disuse, we shall see that a head, eyes, 

 etc., would in fact have been of very little use to molluscs of the second 



