INFLUENCE OF ENVIRONMENT 117 



the impression received, penetrates everywhere and passes through 

 any medium, including even the densest bodies : it follows that every 

 animal, belonging to a plan of organisation of which hearing is an 

 essential part, always has sonie_oppQrtunity, for ^the_ exercise of this 

 organ wherev-er-itjiiay hve. Hence among the vertebrates we do not 

 find any that are destitute of the organ of hearing ; and after them, 

 when this same organ has come to an end, it does not subsequently 

 recur in any animal of the posterior classes. 



It is not so with the organ of sight ; for this organ is found to 

 disappear, re-appear and disappear again according to the use that 

 the animal makes of it. 



In the acephaHc molluscs, the great development of the mantle 

 would make their eyes and even their head altogether useless. The 

 permanent disuse of these organs has thus brought about their dis- 

 appearance and extinction, although molluscs belong to a plan of 

 organisation which should comprise them. 



Lastly, it was part of the plan of organisation of the reptiles, as of 

 other vertebrates, to have four legs in dependence on their skeleton. 

 Snakes ought consequently to have four legs, especially since they are 

 by no means the last order of the reptiles and are farther from the 

 fishes than are the batrachians (frogs, salamanders, etc.). 



Snakes, however, have adopted the habit of crawling on the ground 

 and hiding in the grass ; so that their body, as a result of continu'ally 

 repeated efforts at elongation for the purpose of passing throtugh 

 narrow spaces, has acquired a considerable length, quite out of pro- 

 portion to its size. Now, legs would have been quite useless to these 

 animals and consequently unused. Long legs would have interfered 



that the air should penetrate to all places to which the substance producing sound 

 actually does penetrate. 



See my memoir On the Substance of Sound, printed at the end of my Hydrogeologie, 

 p. 225, in which I furnished the proofs of this mistake. 



Since the publication of my memoir, which by the way is seldom cited, great efforts 

 have been made to make the known velocity of the propagation of sound in air tally 

 with the elasticity of the air, which would cause the propagation of its oscillations 

 to be too slow for the theory. Now, since the air during oscillation necessarily under- 

 goes alternate compressions and dilatations in its parts, recourse has been had to 

 the effects of the caloric squeezed out during the sudden compressions of the air and 

 of the caloric absorbed during the rarefactions of that fluid. By means of these 

 effects, quantitatively determined by convenient hypotheses, geometricians now account 

 for the velocity witli which sound is propagated through air. But this is no answer 

 to the fact that sound is also propagated through bodies which air can neither traverse 

 nor set in motion. 



These physicists assume forsooth a vibration in the smallest particles of solid 

 bodies ; a vibration of very dubious existence, since it can only be propagated through 

 homogeneous bodies of equal density, and cannot spread from a dense body to a 

 rarefied one or vice versd. Such a hypothesis offers no explanation of the well-known 

 fact that sound is propagated through heterogeneous bodies of very different densities 

 and kinds. 



