v] LATER PROGRESS 133 



from birds. The Rook and the Sparrow are almost 

 omnivorous, and therefore very independent as regards 

 food-supply. A bird like the Swallow is a little more 

 dependent. In its large gape and strong flight it 

 shows a general adaptation for catching small insects 

 on the wing, but as long as they are insects and 

 small and flying, it is content ; it has taken advantage 

 of a common property of many insects, and is de- 

 pendent in no narrow way. Dependent it is, however, 

 and when the insects fail it must migrate. 



Finally, such a bird as the Skimmer (Rhynchops 

 nigra) exhibits a very special adaptation indeed. 

 Darwin (3, p. 137) gives a graphic account of them. 

 " The beak is flattened laterally.. . .It is flat and elastic 

 as an ivory paper-cutter, and the lower mandible, 

 differently from every other bird, is an inch and a 

 half longer than the upper/' When feeding, "they 

 kept their bills wide open, and the lower mandible 

 half buried in the water. Thus skimming the surface 

 . . .they dexterously manage with their projecting lower 

 mandible to plough up small fish, which are secured 

 by the upper and shorter half of their scissor-like 

 bills." This strange bill is without doubt an ex- 

 tremely efficient instrument for catching fish near 

 the surface of the water, but the length of the lower 

 mandible, the very particularity which makes it so 

 efficient for this one purpose, renders it unavailable 

 for any other. The narrow domain where air and 



