DIFFERENTIATION, MIGRATION, AND MIMICRY. 367 



Canarian bird. Besides minor differences, the beak is one-fourth 

 stouter and longer than in the European bird, the tarsus very 

 much stouter and longer, and the back is grey rather than russet. 

 The grey back harmonises with the volcanic dark soil of the rocks 

 of the Canaries, as the russet does with the clay of the plains of 

 England and France. In the Canaries the bird lives under 

 different conditions from those of Europe. It is on the mountain 

 sides and among rocks that the stouter beak and stronger legs are 

 indispensable to its vigorous existence. It is needless to go into 

 the details of many other species. We have here the effect of 

 changed conditions of life in 400 years. What may they not have 

 been in 400 centuries ? We have the result of peculiar food in the 

 Pigeons, and of isolation in all the cases I have mentioned. Such 

 facts can only be supplied to the generaliser and the systematist 

 through the accurate and minute observations of the field-naturalist. 



The character of the avifauna of the Comoro Islands, to take 

 another insular group, seems to stand midway in the differentiating 

 process between the Canaries and the Sandwich Islands. From 

 the researches of M. Humblot, worked out by MM. Milne- 

 Edwards and Oustalet, we find that there are twenty-nine species 

 acknowledged as peculiar ; two species from South Africa and 

 twenty-two from Madagascar in process of specification, called by 

 M. Milne-Edwards secondary or^derived species. 



The little Christmas Island, an isolated rock 200 miles south 

 of Java, only twelve miles in length, has been shown by Mr. Lister 

 to produce distinct and peculiar forms of every class of life, 

 vegetable and animal. Though the species are few in number, 

 yet every mammal and land bird is endemic ; but, as Darwin 

 remarks, to ascertain whether a small isolated area, or a large 

 open area like a continent, has been more favourable for the pro- 

 duction of new organic forms, we ought to make the comparison 

 between equal times, and this we are incapable 'of doing. My 

 own attention was first directed to this subject when, in the year 

 1857-58, I spent many months in the Algerian Sahara, and 

 noticed the remarkable variations in different groups, according 

 to elevation from the sea, and the difference of soil and vegetation. 

 The ' Origin of Species' had not then appeared ; but on my return 

 my attention was called to the communication of Darwin and 

 Wallace to the Linnean Society* on the tendencies of species to 

 * Journ. Linn. Soc, vol. hi. (1859), pp. 45—62. 



