ENVIRONMENTAL CLASSIFICATION 43 



development of carnivores among the insects and myriapods. The 

 earliest tetrapod vertebrates to enter upon terrestrial life, the Am- 

 phibia, are still largely insectivorous. Herbivores appear among the 

 vertebrates at a later stage in their phylogeny, with a few reptiles, 

 a few birds, and a large proportion of the mammals. 



The advantages afforded by terrestrial life are counterbalanced by 

 great disadvantages and dangers, which have called forth special 

 adaptations. The most important difficulty consists in the varying 

 humidity of the air. The humidity reaches the maximum at only a few 

 places and then usually only at special seasons, and is usually far 

 below the saturation point, The humidity of the air, combined with 

 temperature, barometric pressure, rate of wind, and amount of sun- 

 shine, conditions the rate of transpiration. Soft-skinned animals, un- 

 der adverse conditions, will be subject to continued evaporation of 

 their body fluids, and finally to the drying up of the skin and the 

 entire body. The skin is permanently injured by drying, its cells are 

 killed, and important functions, such as skin-breathing and sensory 

 and glandular activity, become impossible. The epithelium of the 

 breathing apparatus is especially subject to this danger. The breathing 

 organ must have a large but delicate surface, which will permit the 

 rapid exchange of oxygen and carbon dioxide. Two groups of animals, 

 the arthropods and the vertebrates, were especially suited for ter- 

 restrial life by the structure of their skin. 



The aquatic arthropods have a solid armor, formed by the thick- 

 ened outer layers of the skin. This armor serves primarily as a frame- 

 work for the insertion of muscles, whereby the effectiveness of their 

 action is notably increased, and secondarily as a protection. This 

 exoskeleton overlies the outer surface of the body, which is a vital 

 condition for an effective protection against evaporation. The echino- 

 derms have an external armor which serves as a muscle-supporting 

 skeleton, but in this group the armor is formed by the deposition of 

 lime in the subepidermal layers of the skin, and it is covered out- 

 wardly by the epidermis, which is thus unprotected against drying 

 should the animal leave the water. Thus in spite of the existence of a 

 skeleton, no echinoderm became adapted to a terrestrial or air- 

 breathing existence. The considerable protection against the drying of 

 the skin, already developed in aquatic arthropods, would have been 

 useless as a step toward terrestrial life without some protection to the 

 breathing organs. A respiratory apparatus, to meet this situation most 

 successfully, should be situated in the interior of the body. Most 

 crustaceans, whose breathing organs take the form of thin-skinned, 

 much-branched evaginations of the body wall, find difficulty in acquir- 



