Chap. VI.] TRANSITIONS OF ORGANIC BEINGS. 221 



fruit, and others with elongated wings which chase in- 

 sects on the wing. On the plains of La Plata, where 

 hardly a tree grows, there is a woodpecker (Colaptes 

 campestris) which has two toes before and two behind, 

 a long pointed tongue, pointed tail-feathers, sufficiently 

 stiff to support the bird in a vertical position on a post, 

 but not so stiff as ia the typical woodpeckers, and a 

 straight strong beak. The beak, however, is not so 

 straight or so strong as in the typical woodpeckers, but 

 it is strong enough to bore into wood. Hence this 

 Colaptes in all the essential parts of its structure is a 

 woodpecker. Even iu such trifling characters as the 

 colouring, the harsh tone of the voice, and undulatory 

 flight, its close blood-relationship to our conunon wood- 

 pecker is plainly declared; yet, as I can assert, not only 

 from my own observations, but from those of the ac- 

 curate Azara, in certain large districts it does not climb 

 trees, and it makes its nest in holes in banks! In cer- 

 tain other districts, however, this same woodpecker, 

 as Mr. Hudson states, frequents trees, and bores holes 

 in the trunk for its nest. I may mention as another 

 illustration of the varied habits of this genus, that a 

 Mexican Colaptes has been described by De Saussure as 

 boring holes into hard wood in order to lay up a store of 

 acorns. 



Petrels are the most aerial and oceanic of birds, but 

 in the quiet sounds of Tierra del Fuego, the Puffinuria 

 berardi, in its general habits, in its astonishing power 

 of diving, in its manner of swimming and of flying 

 when made to take flight, would be mistaken by any 

 one for an auk or a grebe; nevertheless it is essentially 

 a petrel, but with many parts of its organisation pro- 

 foundly modified in relation to its new habits of life; 



