92 HEREDITY [ch. 



less it disappears, and the most obvious cause to 

 assume is lack of use, which, acting cumulatively 

 through many generations, has become hereditary. 

 Such evidence, however, is only presumptive, it does 

 not amount to proof, and on the other side may be 

 adduced the pigments of birds' eggs. Birds which 

 nest in holes or dark places usually have colourless 

 or slightly coloured eggs, while those which lay in 

 open places have eggs more or less matching their 

 surroundings. This appears closely comparable with 

 the condition of skin-colour in fishes and amphibians, 

 and yet it is impossible that the action of light could 

 have any direct efiect on the production of pigment 

 in birds' eggs. That the loss of pigment in each case 

 is connected with its uselessness is probable, but the 

 birds' egg case seems to show that it is not due to 

 * use-inheritance.' 



A second instance of the indirect evidence for the 

 inheritance of acquired characters may be given, that 

 of instinct. Instincts are very similar to firmly rooted 

 habits, and have been regarded as habits which from 

 being performed through many generations have 

 become hereditary. There can be no doubt that, in 

 the higher animals especially, instincts may be rein- 

 forced and perfected by habit, but many cases can be 

 adduced in which it seems impossible that habit has 

 played a part in the evolution of an instinct. Many 

 insects have exceedingly perfect and complex in- 



