THE BASIS OF ADAPTATION 



253 



Although it is generally applicable, there is probably no better 

 illustration of the process than the mammals. Originally terres- 

 trial, they have radiated into four habitats. On the basis of 

 hmb modifications, from the original ambulatory stem fossorial, 

 volant, cursorial and aquatic species have arisen. Such animals 

 as the shrews represent the primitive type. Scansorial animals 

 like the squirrels may well be transitional to volant typ(>s. The 

 bats are the most nearly perfect volant mammals (Fig. 134). 

 The mole is the most nearly perfect fossorial form (Fig, 130). 



Cursorial- 

 Unguligrade 



Limbs 



Volant 

 (Aerial) 



Cursorial-Digitigrade 



Scansorial 

 (Arboreal) 



( Short-limbed, plantigrade, 

 (TS^reitr?aB\ P^ntadactyl, unguiculate 

 ' ^ [stem 



Natatorial 

 (Amphibious) 



Fossorial 

 (Subterranean) 



(Aquatic) 

 Fig. 150. — Diagram of adaptive radiation. (After Lull.) 



From ambulatory forms digitigrade runners like the carnivores 

 arose, and through similar limb posture came the highly specialized 

 unguligrade appendages of animals like the horse. Among aquatic 

 mammals the whale is extreme; many amphiljious forms suggest 

 the possibility of gradual change in this direction. The teeth also 

 show adaptive radiation from the primitive insectivorous stem 

 to herbivorous, carnivorous, omnivorous and ant-eating types, the 

 last with greatly reduced dentition. 



Adaptive Branching. Among insects the bugs (Order Hemip- 

 tera) show many evidences of adaptive branching (Fig. 151). 

 They were originally, no doubt, terrestrial ambulatory species 

 with volant powers. We now have highly aquatic species, which 



