1 8 THE CLASSIFICATION OF BIRDS 



groups according to their mode of life, such as (a) Perchers in 

 trees, (b) Runners on land, (c) Waders in water, and (d) Swimmers 

 on water. This was the basis of the earlier attempts at classi- 

 fication; but recent writers have (doubtless correctly) assumed 

 that of the half-dozen (more or less) subclasses into which 

 the surviving species of Birds may be grouped, several of them 

 may contain families whose ancestors succeeded in establishing 

 themselves in other modes of life than the ancestral tree-perching 

 business, and did it so effectually that in spite of the struggle for 

 existence they contain species which have survived the perils 

 aforesaid in more than one, if not in all, of the avian modes of 

 life. Consequently in the attempt to make a natural classification 

 of Birds it is probably wisest not only entirely to disregard the 

 supposed height of development which has been attained, or the 

 assumed depth of degradation which has been reached, but also 

 to view without alarm the association of runners with swimmers 

 or perchers with waders. 



This point, that the primary divisions of the Class Aves may 

 each contain families of Birds which are typically aerial in their 

 habit?, and consequently appear to be very highly developed, as 

 well as others that have been driven in the struggle for existence 

 to adopt aquatic or terrestrial habits, and consequently appear to 

 be very archaic in their development, is the point upon which I 

 am anxious especially to insist in the present essay. In arrang- 

 ing the sequence of the Families, it must always be remembered 

 that with each Order a fresh series commences, with probably a 

 wide gap between the two. These gaps are not blemishes to be 

 concealed, but facts to be welcomed, and rudely represent the age 

 of the original isolation of the groups. The attempt to make a 

 linear series with the smallest possible gaps between the families 

 is a mistake. 



The result of our investigations may possibly be very far from 

 a discovery of the true system of classification, but it will at least 

 provide us with a workable system a system which, so far as we 

 know, contains no group, small or large, that cannot be diagnosed. 



The diagnosis of a genus, a family, or an order is a list of the 

 characters which are possessed by every species contained in it, 

 but which form a combination not found in any species outside 

 its limits. These characters have presumably been inherited, and 

 the greater the number of them which can be found in any group, 

 the stronger is the diagnosis of that group. 



