SUMMARY OF l PHILOSOPHIE ZOOLOGIQUE? 309 



they have therefore been obliged to make efforts to 

 draw back their too projecting claws, and so, little by 

 little, has arisen the peculiar sheath into which cats, 

 tigers, lions, &c., withdraw their claws when they no 

 longer wish to use them.* 



" We see then that the long-sustained and habitual 

 exercise of any part of a living organism, in conse- 

 quence of the necessities engendered by its environment, 

 develops such part, and gives it a form which it would 

 never have attained if the exercise had not become 

 an habitual action. All known animals furnish us with 

 examples of this.t If anyone maintains that the 

 especially powerful development of any organ has had 

 nothing to do with its habitual use that use has added 

 nothing, and disuse detracted nothing from its efficiency, 

 but that the organ has always been as we now see it 

 from the creation of the particular species onwards 

 I would ask why cannot our domesticated ducks fly like 

 wild ducks? I would also quote a multitude of ex- 

 amples of the effects of use and disuse upon our own 

 organs, effects which, if the use and disuse were 

 constant for many generations, would become much 

 more marked. 



" A great number of facts show, as will be more fully 

 insisted on, that when its will prompts an animal to this 

 or that action, the organs which are to execute it 

 receive an excess of nervous fluid, and this is the 

 determinant cause of the movements necessary for the 

 required action. Modifications acquired in this way 

 eventually become permanent in the breed that has 

 * ' I hil. Zool.,' torn. i. p. 256. f ^ge 257. 



