470 FIELD STUDY IN ORNITHOLOGY. 



effect of changed conditions of life in fonr bnndred years. What may 

 they not have been in fonr lumdred centnrics? We have the resnlt of 

 peculiar food in the pigeons and of isolation in all the cases I have 

 mentioned. Such fa(;ts can only be supplied to the generalizer and 

 the systematist through the accurate and minute observations of the 

 field naturalist. 



The character of theavi-faunaof the Comoro Islands, to take another 

 insular group, seems to stand midway in the dift'erentiating process 

 between the Canaries and the Sandwich Islands. From the researches 

 of M. Ilumblot, worked out by MM. Milne-Edwards and Oustalet, we 

 find that there .are 29 species acknowledged as peculiar; 2 species from 

 South Afiica and 22 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 12 miles in length, has been shown by Mr. Lister to produce dis- 

 tinct 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 fav- 

 orable for the i^roduction of new organic forms, we ought to make the 

 comparison between equal times, and this we are inca])able 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, aud noticed the 

 remarkable variations in difi'erent groups according to elevation from 

 the sea and the difi'erence 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 AVallace to the Linn<ean 

 Society on the tendencies of species to form varieties and on the per- 

 petuation of varieties and s^^cies by means of natural selection. I then 

 wrote {Ibis, ISoO, pp. 420-433): 



" It is hardly possible, I should think, to illustrate this theory better 

 than by the larks and chats of North Africa. In all these, in the con- 

 gen<'rs of the wheatear, of the rock chat, of the crested laik, we trace 

 gradual modifications of coloration and of anatomical structure, deflect- 

 ing by very gentle gradations from the ordinary type, but when we take 

 the extremes presenting the most marked differences. - - - In the 

 desert, where neither trees, brushwood, nor even undulations of sur- 

 face aftbrd the slightest protection to an animal from its foes, a modi 

 tication of colors, Avhicli shall be assimilated to that of the surround- 

 ing country, is absolutely necessary. Hence, without exception, the 

 upper plumage of every bird — whether lark, chat, sylvan, or land 

 grouse — and also the fur of all the small mammals and the skin of all 

 the snakes and lizards, is of the uniform isabellineor sand color. It is 

 very possible that some further purpose may be served by the prevail- 

 ing colors, but this apj^ears of itself a sufticient ex])lanation. Tliere 

 are individual varieties of depth of hue annrng all creatures. In the 

 struggle for life which we know to be going on among all species a very 

 slight change for the better, such as iin])roved means of escai)e from its 

 natural enemies (which would be the effect of an alteration from aeon- 



