THE EVIDENCE FROM DEVELOPMENT. 251 



latter birds, to feed upon flower juices and upon the insects that 

 infest flowers. He emphasises the need for distinguishing clearly 

 between characters or likenesses which are " structural " that is, are 

 part and parcel of an animal's being and those that are purely 

 " adaptive " that is, arise from a similar mode of life, independently 

 of the origin of the species. The former are transmitted from 

 ancestors ; the latter are the products of recent modification. The 

 former indicate the true nature of the animal, because they are part 

 of its inheritance ; the latter often suggest false resemblances due to 

 similarity of habits and not to community of origin. Thus, whilst the 

 humming-birds and swifts possess structural and inherited likenesses, 

 the former and sun-birds are related only through similar adaptive 

 characters. The skull of a cuttlefish, to select another example, is 

 comparable in its functions to that of a low vertebrate animal, but on 

 no theory of nature are these two groups connected together. They 

 have arisen, like the similarities of sun-birds and humming-birds, 

 entirely independently, in respect probably of similar conditions, and 

 not of inheritance from a common ancestor. The inherited characters 

 which mark real resemblances are not, as we have seen, always 

 apparent ; and the adaptive characters through which the life of the 

 species is carried on may entirely mask and conceal them. As 

 Mr. Wallace puts it, we arrive at " the seeming paradox, that the less 

 of direct use is apparent in any peculiarity of structure, the greater 

 is its value in indicating true, though perhaps remote, affinities ; while 

 any peculiarity of an organ which seems essential to its possessor's 

 well-being is often of very little value in indicating its affinity for (to) 

 other creatures." Thus we are led to the conclusion, favoured again 

 by development and its lessons, that the humming-birds " are essen- 

 tially swifts profoundly modified, it is true, for an aerial and 

 flower-haunting existence but still bearing in many important peculi- 

 arities of structure the unmistakable evidences of a common origin." 



