PEOPLING OF LAND AND SEA 193 



toes, and that even the slight resistance of the ground on which 

 they press would make the skin stretch laterally so as to form 

 a web. Thus the web is no accidental product here ; it is 

 linked with a mode of life which the animal only accentuates 

 when it takes to swimming. The movements necessitated by 

 swimming have the same consequences everywhere. The better 

 to utilize its strength, the animal immobilizes the bones of its 

 limbs, and the pull on them of the muscles attached to the body 

 during the action of paddling, shortens the bones, while the 

 resistance of the water flattens them, and the web then 

 envelopes all the flat bones immobilized in relation to each 

 other and consequently brought closer together to form a 

 mutual support. Thus the entire foot is transformed into a 

 swimming paddle. This transformation, like the development 

 of the web itself, takes place in the most varied groups : first 

 in the Sauropterygians, 1 the Ichthyopterygians, 2 and certain 

 Mosasaurians — large Reptiles of the Secondary Period — then 

 in marine chelonians. Still later, after having been merely 

 rudimentary in the hind limbs of the Seals, it appears in 

 Halitherium, Zcuglodon, the Sirenians (Dugongs, Manatees), 

 and the Cetaceans. It is so clearly the mechanical conditions 

 of swimming that determine this foot-formation, that among 

 the Birds the Penguin's wing, also transformed into a 

 swimming organ, though it preserves all the essential characters 

 found in the skeleton of a bird's wing, is modified in the same 

 direction and transformed in the same fashion into a swimming 

 paddle. The same limb has thus been successively a foot, a 

 wing, and a swimming organ. 



A parallel series of facts, similarly linked together, leads to 

 the development of flight in the climbing quadrupeds. When, 

 in climbing, they cling to the trunk or branches of a tree, the 

 skin becomes laterally flattened and thrust back on the base 

 of the limbs ; hence a kind of membranous parachute is formed, 

 which can be observed indifferently among Marsupials 

 such as Petaurus, Rodents like Pteromys and Anomalurus, 

 Insectivora like Galeopitheciis, Lemurs like Microcebes, and 

 culminates in the wing of the Bats. A striking instance of 

 this kind of arrangement is seen in certain specificalty 

 climbing Lizards of the Gecko family, which cling closely 



1 Plesiosaurus and related genera. 



2 Ichthyosaurus and related genera. 



