488 THE CHANGING WORLD 



shift respiratory device, effective within the eggshell, or, in the case 

 of mammals, in the uterus of the mother, before it is possible for the 

 lungs of the young individual to take over the task of respiration. 



Curiously, the embryologist often has to describe a different organ 

 from that which the anatomist cites for the accomplishment of the 

 same function in the animal body. An adult anatomical structure 

 over and over again succeeds a transitory embryonic forerunner. 

 Thus, temporary nephroi are followed by permanent kidneys ; downy 

 lanugo is replaced by hair, more or less permanent ; the gauzy em- 

 bryonic covering of epitrichium gives way to the adult skin ; there is 

 a succession of teeth ; the intestine replaces the yolk sac ; the primi- 

 tive vitelline circulation gives over its temporary emergency service as 

 the systemic circulation arises ; the two-chambered, fishlike, embry- 

 onic heart of the mammal becomes replaced by the three-chambered 

 amphibian stage before the final four-chambered heart is established ; 

 while for the embryonic vertebrate skeleton, patterned largely in 

 cartilage, there is eventually substituted a more efficient bony frame- 

 work. 



All these illustrations and many more indicate adaptations to 

 adult life, following the different preliminary conditions imposed by 

 embryonic existence. 



Physiological Adaptations 



When for any reason one kidney is removed, or put out of com- 

 mission, the remaining kidney assumes the double task and increases 

 correspondingly in size. This is a physiological adaptation. 



The apparatus of the sweat glands is a physiological device enabling 

 mammals to adjust themselves to the greater variation in tempera- 

 ture which occurs on land, as contrasted with that to which sweatless 

 water animals are exposed. 



A grasshopper, with its large immovable compound eyes facing 

 everywhere except below where the mouth is located, is not able 

 to see the food that it is eating, so tactile palps, that are sensory 

 modifications of the mouth-parts, become adapted to function in- 

 stead of eyes in the examination of food. 



Darwin cites the strange case of a certain species of parrot in New 

 Zealand which, after the introduction of large herds of sheep into 

 its habitat and after somehow getting the taste of blood, gave up 

 its former vegetarian habit of life and became a murderous, blood- 

 thirsty carnivore, living entirely on the flesh of sheep. 



