1074 LIFE : OUTLINES OF GENERAL BIOLOGY 



throws no direct light on the evolution of the gizzard in birds that 

 live on hard food. 



A very effective "stomach" is found in the cud-chewing ruminants, 

 where the lower end of the gullet or oesophagus has been specialised 

 to form the rumen or paunch, the reticulum or honeycomb, and the 

 psalterium or manyplies, the true stomach, with its gastric glands, 

 being represented by the fourth chamber, the abomasum or reed. 



The right ventricle of the heart of the ptarmigan, which lives at 

 high altitudes, is distinctly stronger than that of its near relative 

 the willow-grouse, which does not ascend so high. This is an easy 

 adaptation to account for, since individual inborn variations in the 

 muscularity of the heart are well known and frequent. The fish called 

 Anableps that lives in the estuaries of Brazil and Guiana has the 

 habit of swimming at the surface with its eyes half out of water; the 

 upper half of the eye is adapted for vision in air; the lower for vision 

 in water; the curvature of the two parts being different. This is a 

 difficult adaptation to account for. Mr. J. T. Cunningham writes: 

 "It seems to me that we have no reason to suppose that the required 

 variations ever occurred until the ancestors of Anableps took to 

 swimming with their eyes half out of water." His Lamarckian 

 interpretation is that the peculiar habit directly brought about 

 modifications in the eye which became by hereditary summation 

 a racial character. This is a good case, and Mr. Cunningham has an 

 expert knowledge of fishes; but on the other side it may be pointed 

 out (i) that there has been relatively little investigation of germinal 

 variation of the eyes of fishes, (2) that we have little warrant for 

 supposing that such a remarkable peculiarity of the lens could arise 

 as the direct result of the peculiar habit, and (3) that we cannot 

 exclude the possibiUty that Anableps had a germinal or constitu- 

 tional peculiarity of the eye which led it to take to its peculiar 

 method of swimming at the surface, where the weakness might 

 become a source of strength. It may be fairly said, perhaps, that this 

 interpretation is just as far-fetched as the other. The fact is that we 

 do not know. 



Developmental Adaptations. — Besides functional adaptations 

 by hundreds, say from warm-bloodedness to heliotropism, and 

 structural adaptations, such as the viper's poison apparatus, or the 

 Venus Fly-trap, among thousands, it makes for clearness to recog- 

 nise another great group of developmental adaptations; which we 

 mentioned in connection with life-histories. We refer to cases like 

 the suppression or lengthening of an arc in the life-history in relation 

 to particular surroundings or seasonal conditions. Thus in most 

 freshwater animals, except insects, there is a suppression of larval 

 stages which lessens the risk of the creatures being swept away. 

 Similarly it might be said that the extraordinary rapidity of larval 



