402 External Changes in Birds, 



tinct plans, upon one or other of which all animals are orga- 

 nised, and which cannot, any more than the last, be confounded, 

 in any instance, one with another, however in particular 

 cases these too may approximate : of which presently. Every 

 vertebrate animal is, therefore, allied to every other vertebrate 

 animal by what, to specify by numbers, may be expressed as 

 three degrees of affinity ; whereas it is physiologically related 

 to every member of the Annulosa, and other invertebrate 

 classes, by only two degrees, its affinity with plants being 

 reckoned as one ; the proportions of these numbers towards 

 each other pretty accurately denoting the value of these de- 

 grees ; two being double one, three exceeding by half two, 

 &c. Animals of the same subclass, as different mammifers, 

 or birds, or reptiles, are, of course, related to each other by 

 four degrees of affinity ; those of the same order by five, and 

 so on; the number of these several degrees increasing in 

 proportion to the number of subordinate successive types 

 upon which different species are alike organised, and of which, 

 successively, they are modifications, not combinations of dif- 

 ferent ones, in the last case any more than in the first. 

 Every modification of every successive type is thus rudiment- 

 ally different from the most approximate modifications of 

 every other equivalent type, or superior type, to which it does 

 not appertain ; and this is the same conclusion to which I 

 have been irresistibly led from consideration of various phe- 

 nomena connected with the changes of plumage which take 

 place in birds. As every species is perfectly and essentially 

 distinct and separate from every other species, so, except in 

 a retrograde direction, are the successive typical and subtypical 

 plans upon which they are severally organised, however similar 

 the latter may in some instances be, as are also the former. 

 It is unnecessary to enter here upon any remarks on hybrids, 

 as further elucidatory of the precise nature of affinity : it is 

 well known that these can only be produced within a certain 

 physiological range, and that their degree of fertility (paired 

 with individuals of pure blood) is in proportion to the degree 

 of affinity between the parent species. 



By the term approximation, I must be understood to signify 

 those modifications of particular types, which, adapted to in- 

 termediate modes of life, very commonly more or less resemble 

 (in consequence of this adaptation) species which are orga- 

 nised on other and different types. I have already had occa- 

 sion to mention certain extreme modifications of the corvine 

 or omnivorous type of perching birds, which are close ap- 

 proximations towards the fringillidous type (as Aglaius and 

 other finch-like Sturnidse, Amm6dramus, and ^41auda) ; the 



