492 



NA TURE 



[September 21, 1893 



the blue titmouse. Our familiar resident is replaced along the 

 coast of North Africa by a representative species, Pai-us ultra- 

 viarinus, differentiated chiefly by a black instead of a blue cap, 

 and a slate-coloured instead of a green back. The titmouse of 

 Lanzarote and Fueitaventura is barely separable from that of 

 Algeria, but is much smaller and paler, probably owing to 

 scarcity of food and a dry desert climate. Passing, loo miles 

 further to sea, to Grand Canary, we find in the woods and forests 

 a bird in all respects similar to the Algerian in colour and di- 

 mensions, with one exception — the greaier wing coverts of the 

 Algerian are lipped with white, forming a broad bar when the 

 wing is closed. This, present in the P'uertaventura form, is re- 

 presen'ed in the Canarian by the faintest white tips, and in the 

 birds from the next islands, Tenerife and Gomera, this is alto- 

 gether absent. This form has been recognised as Panis 

 tenerifce. Proceeding to the north-west outermost island, Palma, 

 we find a very distinct species, with different proportions, a 

 longer tail, and white abdomen instead of yellow. In the 

 Ultima Thule, Hierro, we find a second very distinct species, 

 resembling that of Tenerife in the absence of the wing bar and 

 in all other respects, except that the back is green like the 

 European, instead of sla'e as in all the other species. Thus we 

 find in this group a uniform graduation of variation as we proceed 

 further from the cradle of the race. 



A similar series of modifications may be traced in the chaffinch 

 [Fringilla], which has been in like manner derived from the 

 North African F spodiogena, and in which the extreme varia- 

 tion is to be found in the westernmost islands of Palma and 

 Hierro. The willow wren {PhyUoscofiis trcchilus), extremely 

 numerous and resident, has entirely changed its habits, though 

 not its plumage, and I have felt justified in distinguishing it as 

 Ph. foitunatus. In note and habits it is entirely different from 

 our bird, and though it builds a domed nest it is always near the 

 top of lofty trees, most frequently in palm-trees. The only ex- 

 ternal difference from our bird consists in its paler tarsi and 

 more rounded wing, so that its power of flight is weaker, but, 

 were it not for the marked difference in its habits and voice, I 

 should have hesitated to differentiate it. In the kestrel and the 

 great spotted woodpecker there are differences which suggest 

 incipient species, while the forests of the wooded western islands 

 yield two very peculiar pigeons, differing entirely from each 

 other in their habii«, both probably derived from our wood- 

 pigeon, but even further removed from it than the Columba 

 trocaz of Madeira, and, by their dark chestnut coloration, sug- 

 gesting that peculiar food, in this case the berries of the tiee 

 laurel, has its full share in the differen'iation of isolated forms. 

 If we remember the variability of the pigments in the food of 

 birds, and th; amount absorbed and transferred to the skin and 

 plunage, the variability in the tints and patterns of many animals 

 can be more readily understood. 



One other l)ird deserves notice, the Caccal'is, or red-lesged 

 partridge, for here, and here alone, we have chronological data. 

 The Spaniards introduced Caccabis ruja into Canary, and C. 

 pelrosa into Tenerife and Gomera, and they have never spread 

 from their respective localities. Now, both species, after a 

 residence of only 400 years, have become distinctly modified. 

 C. riifa was introduced into the Azores also, and changed ex- 

 actly in the same manner, so much so that Mr. Godman, some 

 years ago, would have described it as distinct, but that the 

 only specimen he procured was in moult and mutilated, and his 

 specimen proved identical with the Canarian bird. Besides minor 

 differences, the back is one-fourth stouter and longer than in the 

 European bird, and the tarsus very much s'outer and longer, 

 and the back is gray rather than russet. The gray back har- 

 monises 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 diflferent condi- 

 tions from those of Europe. It is on the mountain sides and 

 among rocks that the stouter beak and stronger legs are indis- 

 pensable 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 genera- 

 liser and the systematist through the accurate and minute obser- 

 vations of the field naturalist. 



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

 another insular group, seems to stand midway in the differentiat- 

 ing process between the Canaries and the Sandwich Islands. 



NO. 1247, VOL. 48I 



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 specifica- 

 tion, 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 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 re- 

 marks, to ascertain whether a small isolated area, or a large open 

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

 tion 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 s lil and vegeta- 

 tion. The '• Origin of Species " had not then appeared ; but on 

 my return mv attention was called to the communication of 

 Darwin and Wallace to the Linnean Society on the tendencies 

 of species to firm varieties, and on the perpetuation of varieties 

 and species by means of natural selection. I then wrote (/&> 

 1859, pp. 429-433) : " 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 congeners of the whi;atear, 

 of the rock chat, of the crested lark, we trace gradual modi- 

 fications of coloration and of anatomical structure, deflecting hy 

 very gentle gradations from the ordinary type, but, when vetake 

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

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

 surface afford the slightest protection to an animal from its foes, a 

 modification of colour, which shall l)e assimilated to that of the 

 surrounding country, is abs'lutely 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 isabelline or sand-colour. It is very possible |hat 

 some further purpose may be served by the prevailing colours, 

 but this appears of itself a suflicient explanation. There are 

 individual varieties of depih of hue among all creatures. In the 

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

 species, a very slight change for the belter, such as improved 

 means of escape from its natural enemies (which would be the 

 effect of an alteration from a conspicuous colour to one res^emb- 

 ling the hue of the surrounding objects), would give the variety 

 that possessed it a decided advantage over the typical or other 

 forms of the species. . . . To apply the theory to the case 

 of the Sahara. If the Algerian Desert were colonised by a fei» 

 pairs of cresred larks — putting aside the ascertained fact of the 

 tendency of an arid, hot climate to bleach all dark colours — we 

 know that the probability is that one or two pairs would be 

 likely to be of a darker complexion than the others. These, 

 and such of 1 heir offspring as most resembled them, would be- 

 come more liable to capture by their natural enemies, hawks 

 and carnivorous beasts. The lighter-coloured ones would enjoy 

 more or less immunity from such attacks. Let this state of 

 things continue for a few hundred years and the dark-coloured 

 individuals would be exterminated, the light-coloured remain 

 and inherit the land. This proces.s, aided by the above- 

 mentioned tendency of the climate to bleach the coloration 

 still more, would in a few centuries produce the Galerida abys- 

 sinica as the typical form ; and it must be noted that between 

 it and the European G. crUtata there is no distinction but that 

 of colour. 



" But when we turn to Galerida isabellina, G. arenicola, and 

 G. inacrorhyncha, we have differences, not only of colour, hot 

 of structure. These differences are most marked in the form of 

 the bill. Now, to take the two former first, G. arenicola has a 

 very long bill, G. isabellina a very short one ; the former resorts 

 exclusively to the deep, loose, sandy tracts, the latter haunts 

 the hard and rocky districts It is nianiftst that a bird whose 

 food has to be sought for in deep sand derives a great advantage 

 from any elongation, however slight, of its bill. The other, 

 who feeds among stones and rocks, requires sttcnglh rather than 

 length. We know that even in the type species the size of the 

 bill varies in individuals— in the lark as well as in the snipe. 

 Now, in the desert, the shorter-billed varieties would undeijo 

 comparative ditficully in fir.ding food where it wcs not abundant, 

 and ccrstquently would not be in such vigorous condition as 



