THE EVIDENCE FROM DEVELOPMENT. 



249 



adopt Dr. Weismann's view of the Axolotl as a case of reversion from 

 Amblystoma to its lower stage, or whether we -regard the Axolotl 

 races as representing larval forms permanently settled to form a 

 distinct race and species, the influence of surroundings in evolving 

 new forms of life receives an apt illustration. Arrest of development, 

 an altered way of life, change of surroundings, and allied conditions, 

 are seen to operarte powerfully on animals and plants; and are found 

 to -effect exactly those changes in the constitution of the living world 

 which evolution postulates, and which meet the biologist at every turn. 

 One typical case of similarity in early development, as suggestive 

 of a near or common origin, from the group of birds, may merit 

 mention in the present chapter. Mr. A. R. Wallace points out in an 

 interesting fashion how the humming-birds (Fig. 166) of the New 

 World, placed of old side by side with 

 the Old- World sun-birds (Fig. 168), 

 were, in 1850, separated from the latter 

 forms and placed by Prince Lucien 

 Bonaparte near the swifts (Fig. 167) in 

 his system of classification. That this 

 arrangement was correct that is, was 

 one based upon natural affinities, and 

 not merely upon superficial resem- 

 blanceis easily provable. Thus the 

 breast-bone of a humming-bird and 

 that of a swift are marvellously like. 

 It is not notched behind in either bird, 

 whilst this bone in the sun-bird bears 

 two depressions. In the colour and 

 number of the eggs, swifts and hum- 

 ming-birds agree, and they also present 

 close resemblances in the arrangement 

 of their feathers. Both have ten tail 

 feathers and sixteen true quill feathers ; 

 and in both the first quill is longest. 

 But whilst the bill of a swift is short, 

 broad, and flat, the gape wide, and the tongue flat, the humming- 

 birds have a long, slender, cylindrical bill, and a tubular tongue, 

 which can be protruded to a great extent, and which is used for 

 drinking up the nectar of flowers. We shall allow Mr. Wallace to 

 tell us of the striking resemblance between these two groups of birds, 

 revealed by a study of the humming-bird at an early stage of growth. 

 " When on the Amazon," says Mr. Wallace, " I once had a nest 

 brought me containing two little unfledged humming-birds, apparently 

 not long hatched. Their beaks were not at all like those of their 

 parents, but short, triangular, and broad at the base just the form 



FIG. 166. HUMMING-BIRD. 



