FIELD STUDY IN ORNITHOLOGY. 469 



back is greeu, like the European, instead of slate, as iu all tlio 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 chafifinch {Frin- 

 gilla), which has been in like manner derived from the North African 

 F. sjjodiogena, and in which the extreme variation is to be found in the 

 westernmost islands of Talma and Hierro. The willow wren {Phi/llos- 

 copus trochilus), extremely numerous and resident, has entirely changed 

 its habits, though not its plumage, and I have felt justified in distin- 

 guishing it as Ph.forfioiatus. In iijote 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 external 

 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 his habits and voice, I should have hesitated to 

 differentiate it. Jn the kestral 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, differ- 

 ing entirely from each other in their habits, both probably derived 

 from our woodpigeon, 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 tree laurel, 

 has its full share in the differentiation of isolated forms. If we remem- 

 ber the variability of the pignu'uts in the food of birds and the amount 

 absorbed and transferred to the skin and plumage, the variability iu 

 the tints and patterns of many animals can be more readily under- 

 stood. 



One other bird deserves notice, the Caccahis, or red-legged partridge, 

 for here, and here alone, we have chronological data. The Spaniards 

 introduced Caecahis rnfa into Canary and C. pcfrosa into Teneriffe and 

 Gomera, and they have never spread from their respective localities. 

 Now, both species, after a residence of only four hundred years, have 

 become distinctly modified. C. rufa was introduced into the Azores 

 also, and changed exactly in the same manner, so much so that Mr. 

 Godmau, some years ago, would have described it as distinct, but that 

 the only specimen he procured was iu molt and mutilated, and his 

 specimen proved identical with the Cauariau bird. Besides minor dif- 

 ferences, the back is one-fourth stouter and longer than in the P^uro- 

 pean bird, and the tarsus very much stouter and longer, and the back 

 is gray rather than russet. The gray back harmonizes with the vol- 

 canic dark soil of the rocks of the Canaries, as the russet does with 

 the clay of the plains of England aud 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 iudispensiible to its vigorous existence. It is need- 

 less to go into the details of many other species. We have here the 



