188 PROCEEDINGS OF THE ACADEMY OF 



nearly) of canescens ; 3, a smaller, more regularly conic bill than linarius, 

 generally rather dusky than yellow, and with extremely heavy plumules ; 4, 

 remarkably small feet, produced mainly by absolute shortening of the toes. 

 We have here a unique assemblage of characters ; the modification that this 

 per centum of ^'Egiolhiis have undergone is not just like that of any other. Cli- 

 mate, apparently, has, as in the case of canescens, done what might have been 

 expected in respect to color: it has bleached the tints into the semblance of 

 those of canescens, taken away the stripes from the rump, leaving this largely 

 and purely white, and reduced those on the sides to a minimum; whitened the 

 edges of the wing and tail feathers ; made the rosy paler, and thickened the 

 nasal plumules. Latitude, on the other hand, has not efiected any perceptible 

 increase in size. This is curious, viewed beside the case of ro stratus, in which 

 size is increased, but color unaffected. Superadded to these changes is the 

 singular modification of the feet. 



As is well known, absolute size of these members, and relative proportions 

 of the toes to each other, as well as to the length of tarsus, are among the 

 more constant features that birds present. Only a narrow margin seems to be 

 allowed, in the same species, for variation in these respects. In fact, looking 

 over the annals of ornithology, one is struck with the number of proposed 

 species called " longipes," " brevipes," " brachydactylus," etc., that have sub- 

 sequently been shown to be only accidental, or very partial modifications, not 

 holding good as a general rule. There are a great number of synonyms of this 

 particular class, needing no more than this allusion. At the same time, it is 

 equally well known that certain closely allied birds do really difl'er in precisely 

 tills particular, — sometimes with other peculiarities superadded, sometimes 

 with scarcely any, or none. A more or less decidedly terrestrial or arboreal 

 mode of life may reasonably be presumed, if not logically inferred, to have 

 something to do with this change. The case is better illustrated in comparing 

 allied genera. Thus Mimus is an eminently bush- and tree-living genus ; the 

 species of its nearest ally, Harporhi/nchtis, spend much of their time on the 

 ground, walking and scratching among leaves, &c. The feet of the latter are 

 correspondingly larger and stronger than those of the former. The same is 

 the case regarding P^jo/Zo, as compared with other allied genera less eminently, 

 or scarcely, terrestrial in habit. If such modification can be traced in this 

 grade of forms, I see no reason why it should not be exhibited, in however 

 much less degree, between congeneric species that differ in a more or less de- 

 cidedly arboreal mode of life. Admitting, then, possible modifications of the 

 feet, in specific as well as generic grades, — modifications correspondent to the 

 nature of the foot-hold that the birds habitually take, — there seems no reason 

 why the argument by analogy should not be carried a step further, and made 

 to include possible results from a difference in the kind of trees or bushes, or 

 the kind of ground that arboreal or terrestrial species respectively frequent. 

 Terrestrial sparrows of muddy situations will probably be found to have some 

 modifications of the feet not shared by those of sandy deserts, or of rocky 

 gorges. Rush-sparrows, as Ammodromi, that climb up perpendicular swaying 

 stems, have not the same feet as their nearest allies among bush-sparrows that 

 habitually rest upon horizontal and less yielding twigs. These are merely 

 illustrations in point of what I wish to propose, — namely, that the small feet 

 of ^4. exi/ipes may be due to a difference in the size, texture, &c., of the trees or 

 bushes that they habitually frequent, or are really confined to, as compared 

 with the greatly varying range of footholds that linarius, in its extensive move- 

 ments, necessarily takes. It does not seem irrational to suppose that the 

 stunted resinous conifers that form so marked a feature of the northern flora 

 may have produced, in the course of time, the modification that is now wit- 

 nessed. However well or ill grounded the suggestion may be, it is at least a 

 fair inference ; and, at any rate, I know of no other assignable cause for the 

 observed fact. 



As intimated when canescens was spoken of, it is interesting to note that the 



[Oct. 



