CONVERGENT EVOLUTION 447 



tribe will be discussed, and it would be well to bear in nniind 

 these facts. 



It would be well now to take a few cases of convergence 

 from what we may call the " aerial " types on the one hand, and 

 the aquatic types on the other. 



Of the first kind the Swallows and Swifts furnish very strik- 

 inig examples. Representatives of both happily occur in Great 

 Britain, and are indeed to be counted among our commonest 

 summer visitants. So nearly alike are these two forms that 

 by the ordinary observer they are merely regarded as different 

 species of a common type. As a matter of fact the nearest 

 allies of the Swifts are the Night-jars, to which certain exotic 

 forms bear a close likeness ; while the Swallows are members 

 of a totally different order. What are their nearest allies is 

 still a moot point, but they belong to the great group known 

 collectively as the "Passeres"'. Too well known to need a 

 detailed description here, it will be sufficient to draw attention 

 to the fact that this resemblance of quite unrelated forms is due 

 to adaptation to the requirement of catching living insect prey 

 on the wing. In the Night-jars, which are crepuscular, the 

 mouth is of enormous size ; in the allied Swifts and the unre- 

 lated Swallows the mouth is also of great size, but is not so 

 exaggerated a feature. All these capture insect prey while 

 travelling at great speed through the air. The legs in all these 

 forms have become exceedingly reduced, so much so that 

 some of the Swifts find it extremely difficult to take flight 

 from a level surface, sometimes indeed impossible. The wings, 

 on the other hand, have become more or less pointed and 

 ribbon-like when extended, but they differ much one from 

 another when closely examined. Thus the Swift has an ex- 

 traordinarily short arm and forearm and a hand of great 

 length ; in the Swallow, on the other hand, the arm and fore- 

 arm are not conspicuously short, neither is the hand specially 

 long. 



To the Swifts and Swallows, in times past, have been added 

 a third group known as the Pratincoles. This alliance was made 

 on the supposition that, bearing a very real resemblance to 

 these birds, both in form and habit, they must of course be 

 related. We know now that this is not the case. The Pratin- 

 coles have been proved to be aberrant members of the Plover 



