22 THE CLASSIFICATION OF BIRDS 



advantage to be derived from such loss appears possible. No 

 instance of the loss of any other secondary except the fifth is 

 known, nor is any case of partial loss of a secondary to be found 

 except in the family Phasianidce, all the species of which have the 

 first secondary reduced in size. That the fifth secondary should 

 have been lost independently in different groups of birds, or that, 

 having been lost, it should have been re-acquired by the majority 

 of species in one or more groups, is very remarkable. The only 

 other hypothesis capable of explaining the facts is that for some 

 unknown reason the coverts of the fifth secondary have been 

 also lost in some species, and that thus the sole evidence of its 

 ever having been there has been destroyed. Whatever may be 

 the explanation, the fact is undisputed that amongst the birds 

 which have abnormal plantars both quincubital and aquincubital 

 species are to be found, though the latter are comparatively few 

 in number. 



Based upon these characters, the simplest diagnosis of the five 

 Subclasses of Birds is as follows : 



SPH^ENISCOMORPH^E. Aves with none of the feathers of the 

 vring differentiated into quills, and the bones of the forearm all 

 flattened. 



PELAEGOMORPHyE. Aves with normal plantar tendons, but 

 with the fifth secondary absent. 



CORACIOMORPH.E. Aves with abnormal plantar tendons, and, 

 as regards the yOung, altrices.* 



* Recent investigations, though some of them are based upon the observation 

 of birds in confinement, seem to suggest that the condition of the young at birth 

 is a character that may vary in not very distantly related groups. It seems to 

 be a remarkably constant character on the whole, but there are several cases of 

 the apparently independent acquirement of one or other condition. 



If the earliest birds were arboreal in their habits, it is most likely that they 

 built in trees, and laid their eggs in places where it would be dangerous or 

 difficult for young birds to leave the nest until they were able to fly, so that it 

 was manifestly advantageous to them to be altrices. Assuming this condition 

 of the young to be the primitive one, the following groups have acquired the 

 habit of remaining in the egg until they are sufficiently developed to run about 

 in pursuit of food or in avoidance of danger : 



Anseres. 



Ealliformes (with the possible exception of the Heliornithidce). . 



All the Charadriiformes except the Alcidce and Cdumbida. 



Turnices and Crypturi. 



All the Galliformes, with the probable exception of the Fodicida and Opistho- 

 comidce. 



Droma?omorpbae. 



