ADAPTATIONS 435 



allied species, Levaillant's Darter, the proventricular cud of the 

 stomach is not specially remarkable ; but there is a similar 

 pyloric chamber guarded by hairs, which, however, have a quite 

 different arrangement, forming a conical plug immediately in 

 front of the entrance to the gut. Curiously enough, this plan 

 of guarding the gateway to the intestine has been adopted also 

 by the South American "Turkey-buzzard" {Cathartes aura). 

 Here the hairs are of various lengths, and range in colour from 

 black to greenish-yellow. No other birds of prey seem to have 

 found it necessary to adopt a similar device, just as no other 

 fish-eating birds have copied the Darters in this respect. The 

 independent development of a structure so peculiar in birds 

 which are not genetically related is a point of considerable 

 interest. 



The presence of these hairs in the gizzard recalls the fact 

 that in the Common Cuckoo the gizzard is lined throughout with 

 hairs, but these prove on examination to be purely extraneous, 

 being the matted hirsute covering of the caterpillars — " woolly 

 bears" — and other species on which this bird feeds. No other 

 bird can eat these caterpillars on account of the formidable 

 nature of their covering, yet structurall}^ nothing has yet been 

 discovered in the structure of the stomach of the Cuckoo which 

 explains why these birds, and these alone, are able to make 

 such caterpillars their special prey. 



Certain species of Tanagers, a group of birds hardly to be 

 distinguished from Finches, have contrived to dispense with 

 a gizzard altogether. The species in question belong to the 

 Genera Euphonia and Chlorophonia. In these birds the 

 glandular proventriculus, or fore-stomach, opens into the gut, 

 from which, however, it is separated by a narrow non-glandular 

 zone representing the last of the gizzard. This peculiar ar- 

 rangement at present furnishes an unsolved problem, for these 

 birds do not appear to differ in the nature of their food from 

 other Tanagers and Finches with normal gizzards. 



Passing now to the intestines we shall find that these also 

 present a number of puzzling variations for future workers to 

 interpret. For much of our knowledge on this head we are 

 indebted to the philosophical investigations of Dr. Chalmers 

 Mitchell. We arc concerned here, however, only with such 

 features as bear upon the problem of adaptation. 



