Chap. XXIV. EFFECTS OF USE AND DISUSE. 287 



and the shells of nuts. Here we have extremely hard woody 

 tissue without the possibility of any movement, and without, 

 as far as we can see, any other- directly exciting cause ; and as 

 the hardness of these parts is of manifest service to the plant, 

 we may look at the result as probably due to the selection of 

 so-called spontaneous variations. Every one knows that hard 

 work thickens the epidermis on the hands ; and when wo 

 hear that with infants, long before birth, the epidermis is 

 thicker on the palms and soles of the feet than on any other 

 part of the body, as was observed with admiration by Albinus. 1 " 

 we are naturally inclined to attribute this to the inherited 

 effects of long-continued use or pressure. We are tempted to 

 extend the same view even to the hoofs of quadrupeds ; but 

 who will pretend to determine how far natural selection may 

 have aided in the formation of structures of such obvious 

 importance to the animal ? 



That use strengthens the muscles may be seen iu the limbs of 

 artisans who follow different trades; and when a muscle is 

 strengthened, the tendons, and the crests of bone to which they are 

 attached, become enlarged ; and this must likewise be the case with 

 the blood-vessels and nerves. On the other hand, when a limb is 

 not used, as by Eastern fanatics, or when the nerve supplying it 

 with nervous power is effectually destroyed, the muscles wither. 

 So again, when the eye is destroyed the optic nerve becomes 

 atrophied, sometimes even in the course of a few months. 19 The 

 Proteus is furnished with branchiae as well as with lungs : and 

 Schreibers 20 found that when the animal was compelled to live in 

 deep water, the branchiae were developed to thrice their ordinary 

 size, and the lungs were partially atrophied. When, on the other 

 hand, the animal was compelled to live in shallow water, the lungs 

 became larger and more vascular, whilst the branchiae disappeared 

 in a more or less complete degree. Such modifications as these are, 

 however, of comparatively little value for us, as we do not actually 

 know that they tend to be inherited. 



In many cases there is reason to believe that the lessened use of 

 various organs has affected the corresponding parts in the offspring. 

 But there is no good evidence that this ever follows in the course 

 of a single generation. It appears, as in the case of general or in- 



18 Paget, ' Lectures on Pathology,' p. 10) a curious account of the atrophy 

 vol. ii. p. 209. of the limbs of rabbits after the de- 



19 Muller's ' Phys.,' Eng. translat., struction of the nerve. 



pp. 54, 791. Prof. Reed has given 2 ° Quoted by Lecoq, in ' Geograph. 



(' Physiological and Anat. Researches, ' Bot.,' torn, i., 18 j4, p. 182. 



