418 AVES. 



much-vexed question. As stated above they are reptihan in 

 their affinities, but there are no transitional forms connecting 

 them with reptiles. They make their appearance in the Upper 

 Jurassic, and the earliest bhd known— Archaeopteryx—i^resenta 

 almost all the features of specialisation characteristic of living 

 forms. Wliatever may have been the origin — whether from 

 reptilian or from proreptilian creatures — of their peculiar type 

 of structure, the avine is the only vertebrate organisation which 

 has enabled its possessors to make a complete conquest of the 

 air and to fill it with a countless number of inhabitants. But 

 although a new world has been opened to them, their organisa- 

 tion, except in trivial details, has not responded to the infinite 

 diversity of the new environment. This is a fact not without 

 significance to the student of organic evolution, and one to which 

 we shall return when considering that subject in its wider aspects. 

 Meanwhile we may note that the achievement of the power of 

 flight by an animal of the bulk of a bird has been a rare phe- 

 nomenon in nature ; so rare indeed that birds are practically 

 without competitors in their aerial surroundings. This may 

 account for the small amount of structural modification met 

 with in the class, but on the other hand it suggests that the 

 adjustments of machinery necessary to enable an animal of the 

 weight of a fair-sized bird to fiy with ease and certainty are so 

 delicate and minute that no departure from them is possible, a 

 suggestion which receives some corroboration from the con- 

 sideration that the most remarkable of the not very remarkable 

 deviations from the normal avine type are presented by birds 

 which have lost the power of flight and have become adapted 

 exclusively to a terrestrial or aquatic life ; we refer to the 

 Ratitae and the penguins. It is interestmg to note that rismg 

 from the ground or water appears to have been one of the greatest 

 difficulties which nature has had to overcome in enabling birds 

 to fly. Some of the strongest and most enduring flyers ex- 

 perience a difficulty m this respect, e.g. the albatrosses. A further 

 point to be noticed is that the power of flight appears to be in- 

 consistent with great size and weight. The largest flying birds, 

 e.g. the vulture, bustard, swan, turkey, etc., are not large or 

 heavy animals, and in no case in which considerable size and 

 weight has been attained is the power of flight present. 

 The flight of birds is entirely effected by the anterior extre- 



