Chap. XXIT. EFFECTS OF USE AND DISUSE. 297 



or thorns of trees, and the shells of nuts. Here we have ex- 

 tremely hard woody tissue without the possibility of any move- 

 ment to cause exudation, 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 we hear that with infants 

 long before their birth the epidermis is thicker on the palms and 

 soles of the feet than on any other part of the body, as was ob- 

 served with admiration by Albinus, 18 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 in 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 be- 

 comes 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 

 branchise 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 modifi- 

 cations 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 genera- 

 tion. It appears, as in the case of general or indefinite variability, that 

 several generations must be subjected to changed habits for any appreci- 

 able result. Our domestic fowls, ducks, and geese have almost lost, not 



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



vol. n. p. 209. of the lilQbs of rabbits after the de _ 



"Midler's 'Phys.,' Eng. translat., struction of the nerve, 



pp. 54, 791. Prof. Heed has given 20 Q uote d by Lecoq, in < Geograph. 



(' Physiological and Anat. Eesearches,' Dot.,' torn. i. 1854, p. 182. 



