ECOLOGICAL 95 



limbs has not gone nearly so far. Moreover, in the seal-tribe the 

 mothers have to go ashore to bring forth and suckle their 

 offspring. 



Compared with the remodelling of the whale's body, the trans- 

 formation of the penguin or the turtle is relatively slight. The 

 penguin's wings have become stiff oars, but the strong keel on 

 the breastbone is enough to show how far this old-fashioned- 

 looking and flightless bird is from the antique running birds, such 

 as ostriches, where there is no keel at all. The typical clawed limb 

 of the terrestrial Chelonian has become the nail-less flipper of the 

 sea-turtle, and the sea-snakes show an interesting side- to-side flat- 

 tening of the tail, sometimes carried on to the posterior body 

 as well, the better to grip the water in the lateral thrusts of 

 swimming. 



The blubber of a Cetacean is an exaggeration of the subcutaneous 

 deposit of fat that is common in mammals. It makes the creature 

 more buoyant; it conserves the precious animal heat which is apt 

 to be lost in the cold water; it is also a store of calories for evil days. 

 The thick skin and the dense fur, often double in the seals and their 

 relatives, has also a heat-conserving value. 



Since the lung-breathers that return to the sea must inspire dry 

 air at the surface, we can understand the fitness of the whale's 

 power of taking enormously big breaths, of storing oxygenated 

 blood in wonderful networks {reiia mirahilia) of blood-vessels, and 

 of rapidly increasing its stock of red blood corpuscles. And, speaking 

 of respiration, we see great interest in the fact that some marine 

 turtles and sea-snakes have superficial capillary networks in the 

 cavity of their mouth and about their jaws — a quaint return towards 

 gill-breathing, though of course quite distinct. 



The impression of the plasticity of life grows on us as we study 

 the animals that have returned from the land to the sea. See how 

 the sharp points of the seal's teeth are tilted backwards, the better 

 to grip the fish; how the nostrils in Cetaceans can be closed in diving; 

 how the mother-whale can give its baby a big drink of milk at a 

 time; how the kidney often has a very large lobulated excretory 

 surface; how the retrogression of the olfactory area of the brain is 

 associated with the development of something better, and so on. 

 The fact is, that all these animals that have returned to the sea are 

 but specially interesting instances of the universal truth that an 

 organism is a bundle of adaptations — unified. 



AERIAL. — ^The last region to be conquered was the air. Insects 

 were the first to possess it, but it was long before they were followed. 

 The flying-fishes vibrated their fins above the foam as they leapt; 

 the web-footed tree-frogs (Rhacophorus) and various lizards, with the 

 skin spread out on elongated ribs, began to swoop from branch to 



