406 External Changes in Birds, 



with the number of degrees of affinity ; and it is not unusual, 

 too, to find colours or markings, which, in typical forms are 

 scarcely discernible, developed, as it were, in particular mo- 

 difications of those forms, to a considerable extent : yet, in the 

 most approximate modifications of diverse subtypes of one 

 general type, we only find such trivial resemblances of this 

 kind as may be directly traced up to the typical standard 

 from which they both diverge ; whatever other marks of simi- 

 litude these may show being obviously analogous adaptations, 

 rather, to similarities of habit, unaccompanied by those trivial 

 resemblances which imply physiological proximity. Thus, 

 however closely, both in form and colouring, our common grey 

 flycatcher (Muscicapa Grisola) may approximate to some of the 

 smaller Tyrannulae of North America, the mottled character 

 of its nestling garb at once indicates that it is not framed on 

 exactly the same series of successive types ; in a word, that 

 its relation towards these tyrannules must be considered as 

 one of approximation, rather than of direct affinity. It would 

 be easy, in like manner, to illustrate the preceding several 

 positions ; but the limits of the present disquisition will not 

 permit of it. 



It only now remains for me to apply the various facts which 

 I have been endeavouring to establish ; after which I shall 

 commence a minute detail of observations on the moulting of 

 birds. That our systems of classification should be founded 

 on the true affinities of species, rather than upon any arbitrary 

 characters, is now, I believe, admitted on all hands to be the 

 desideratum ; and the true principle on which alone this can 

 be effected is, as it appears to me, sufficiently obvious; 

 though, from our present very imperfect acquaintance with 

 existing species, it must necessarily be a long while yet before 

 our arrangements can be considered at ail final, if, indeed, 

 we can ever hope them to assume that character. 



The true physiological system is evidently one of irregular 

 and indefinite radiation, and of reiterate divergence and rami- 

 fication from a varying number of successively subordinate 

 typical plans ; often modified in the extremes, till the general 

 aspect has become entirely changed, but still retaining, to the 

 very ultimate limits, certain fixed and constant distinctive 

 characters, by which the true affinities of species may be 

 always known; the modifications of each successive type 

 being always in direct relation to particular localities, or to 

 peculiar modes of procuring sustenance; in short, to the par- 

 ticular circumstances under which a species was appointed to 

 exist in the locality which it indigenously inhabits, where 

 alone its presence forms part of the grand system of the uni- 



