318 Proceedings of the Royal Physical Society. 



The older taxononiists certainly proceeded on wrong prin- 

 ciples in their attempts at the classification of birds ; as, in 

 nearly every case, as they selected, as the basis for their 

 systems, just those structural characters that are directly and 

 intimately connected with the well-being of the species as it 

 stands at the present day. In other words, they employed 

 as a basis for classification the characters that have been the 

 latest to be acquired, and that are, in consequence, most likely 

 to undergo rapid changes as the species adapts itself to 

 changes in its environment. Some of these same character- 

 istics have almost certainly been independently evolved by 

 more than one group, simply because they happened to meet 

 their respective requirements. 



A zoological classification, to be really natural, must be an 

 expression of the genetic history of the animals under notice 

 — the higher grades corresponding to the modification dating 

 farthest back in the history of the group, and the lower 

 grades coincidinG^ with the differentiations of later date. 

 Such an ideal classification, at least in the case specially 

 under consideration, is as yet unattainable. All we can do 

 at present is to review every possible feature connected with, 

 birds, and to assign, generally, a low taxonomic value to those 

 characters that are directly connected with the present 

 welfare of the species, or the genus, as the case may be, 

 and to give a liigher rank to any and every feature that 

 is clearly a vestige of a past condition, and has long been, 

 so to speak, hors de comhat, so far as the struggle for exist- 

 ence is concerned. It is such characters as these that 

 may be expected to retain archaic characteristics, and will 

 be, therefore, of value in working out a classification of 

 birds based upon their genetic history. Under this cate- 

 gory should be classed the history of the development of 

 the individuals ; osteological peculiarities, such as those 

 afforded by the character of the palate, of the sternum, or 

 of the nasal bones ; myological facts, such as the presence 

 or the absence of the ambiens muscle, etc. ; visceral, as in 

 the case of the coeca, etc. ; dermal, as in the distribution 

 of the feather tracts, or the nature of the oil-gland. Such 

 features, again, as the relative precocity of the chicks, upon 



