120 THE WONDER OF LIFE 



Fundamental Adaptations. — Prof. Cuenot has noted 

 that there are four adaptations essential to thoroughly 

 terrestrial life. (1) The animal must be able to breathe 

 dry air, either by the skin (as in earthworms), or by some 

 special apparatus, such as the air-tubes of insects, the 

 lung-books of scorpions, the pulmonary chamber of snails, 

 and the true lungs of Amphibians, Reptiles, Birds and 

 Mammals. (2) The animal must be able to resist a con- 

 siderable range of variation in temperature and humidity, 

 and thus we find in terrestrial animals all sorts of cuticular 

 and integumentary structures, such as feathers and hairs, 

 and all sorts of detailed devices for meeting the notable 

 changes in vital conditions that the succession of the 

 seasons involves. Thus hibernation and warm-blooded- 

 ness find their place here as exceedingly effective adapta- 

 tions to terrestrial Ufe. (3) A terrestrial animal wiU tend 

 to have an abbreviated life-history, or in other words a 

 direct development, for the conditions of life on land are 

 not suited for larval stages. The notable exception is in 

 the case of insects, many of which must be called terres- 

 trial, and many of which have intricate life-histories with 

 a great variety of larvae. It will be noted, however, 

 that many insect larvae are very carefully hidden away, 

 that many are specially adapted to be inconspicuous, and 

 that many are pecuHarly protected from possible enemies, 

 e.g. by being unpalatable, by beiag covered with irrita- 

 ting hairs, by exuding repulsive fluids. On the whole, 

 it is safe to say that it is characteristic of terrestrial 

 animals that the young are born or hatched at a very 

 advanced state. What comes out of the egg of a spider 

 or a snail is a miniature of the adult, fully formed. Some 

 species of Peripatus and many insects are viviparous. In 



