98 THE VALUE OF COLOUR 



he fully recognized the community between pro- 

 tection by concealment and more aggressive modes 

 of defence ; for, in the passage of which a part is 

 quoted above, he specially refers to some earlier 

 remarks on p. 226 of his vol. i. We here find 

 that when the oxen were resting by the Juk 

 rivier (Yoke river), on July 19, 1811, Burchell 

 observed ' Geranium spinostim, with a fleshy stem 

 and large white flowers . . .; and a succulent species 

 of Pelargonium ... so defended by the old panicles, 

 grown to hard woody thorns, that no cattle could 

 browze upon it.' He goes on to say, ' In this arid 

 country, where every juicy vegetable would soon 

 be eaten up by the wild animals, the Great Creating 

 Power, with all-provident wisdom, has given to 

 such plants either an acrid or poisonous juice, 

 or sharp thorns, to preserve the species from 

 annihilation . . .' All these modes of defence, 

 especially adapted to a desert environment, have 

 since been generally recognized, and it is very 

 interesting to place beside Burchell's statement 

 the following passage from a letter written by 

 Darwin, Aug. 7, 1868, to Gr. H. Lewes : — 



' That Natural Selection would tend to produce the most 

 formidable thorns will be admitted by every one who has 

 observed the distribution in South America and Africa (vide 

 Livingstone) of thorn-bearing plants, for they always appear 

 where the bushes grow isolated and are exposed to the attacks 

 of mammals. Even in England it has been noticed that all 

 spine-bearing and sting-bearing plants are palatable to quad- 

 rupeds, when the thorns are crushed." 



' More Letters, i. 308. 



