EVOLUTION 1073 



form of body, adapted to moving through holes or in contact with 

 adjacent bodies on all sides, is illustrated by worms, leeches, lam- 

 preys, eels, burrowing blind- worms (Ca^cilians), slow- worms (limbless 

 Lizards), subterranean snakes, and so on, and even so far by 

 elongated sinuous mammals, such as stoats and weasels. 



ADDITIONAL INSTANCES OF ADAPTATIONS 



It is not easy to find a more striking instance of adaptation 

 than that of the African egg-eating snake (Dasypeltis scahra), 

 a weak-bodied creature less than a yard in length, which is able 

 to swallow birds' eggs three times the diameter of the thickest 

 part of its body. The jaws — of course loose-hung, as in all serpents, 

 to aid their wholesale swallowing — have a poor equipment of teeth 

 — ^few and weak. There is the usual serpent way of gripping and 

 loosening, first on one side of the mouth and then on the other; 

 the egg slips intact from the muscular pharynx into the elastic gullet. 

 There it is met by the sharp, tooth-like, and even enamel-tipped(!) 

 spines of a number of anterior vertebrae, which project into the 

 gullet and cut the egg-shells neatly. The result of the structural 

 adaptation is that not a drop of the precious fluid contents of the 

 egg is wasted, and there is a final touch of perfection in the return 

 of the empty shells out of the mouth. What an advance there is in 

 this effective arrangement, as compared with the rough-and-ready 

 way in which other egg-eating snakes break the shell, by pressing 

 their throat against the ground. 



Adaptations of Soft Structures. — One has only to think of 

 organs like the eye or the heart to realise how many distinct adapta- 

 tions may have to be integrated towards the perfecting of a single 

 function. Let us take a simple and familiar case — the gizzard of a 

 grain-eating bird, such as the fowl. Its function is grinding the hard 

 food, and it is adaptive (i) in its great wheel-like muscles, whose 

 contractions bring the walls near one another; (2) in its horny lining, 

 which protects the internal surface from being injured by the pebbles 

 which have been swallowed to serve as grinding-stones ; and (3) in 

 its curvature, which lessens the risk of the food passing into the 

 intestine before it has been ground. This gizzard is a transformed 

 portion of the stomach, but is now non-glandular ; and the problem 

 of its evolution presents no particular difficulty, especially since we 

 know that the stomach of a particular species of bird may show 

 considerable plasticity according to the diet for the time being. Thus 

 the stomach of the herring-gull has a predominantly soft wall in 

 winter, when it is feeding on fish, and a much harder wall in summer, 

 when it is stealing not a little grain from the harvest-fields. This 

 individual plasticity is interesting; yet it is quite possible that it 



VOL. II z 



