52 EXPERIMENTAL EVOLUTION LECT. 



known instance of colour variation, being generally 

 brown, but sometimes blue, and even red in its 

 living state. Leeches offer a large amount of colour- 

 variation ; most Helices do the same, and in fact, it 

 may be said that in all groups of animals varia- 

 tions are met with in the colour of their garment. 

 I refer here merely to occasional variations, for it is 

 well known that a large number of mammals, birds, 

 and other animals offer periodical or seasonal colour 

 variations, especially in northern climates, being 

 brown or grey during the summer, and becoming 

 white during the winter. Such seasonal variations 

 Wallace, in his recent and excellent book on 

 Darwinism, ascribes to natural selection and to pro- 

 tective necessities. Very numerous instances thereof 

 might be adduced ; and Godron in his DcFRspeceet des 

 Races dans les Rtres organises (1859, two volumes), gives 

 a list which might be extended of course of the 

 mammals and birds and other animals which show 

 this seasonal variation, and also a list of animals which 

 offer instances of albinism, melanism, and crythrism. 

 But I maybe allowed to refer to Wallace's Darwinism 

 for all seasonal colour variations, and for the investi- 

 gation of the use and origin of colour generally. 

 As the last named cases of colour variation, such as 

 albinism and melanism, cannot be interpreted in a 

 quite satisfactory manner, we had better leave them 



