THE FLYING-SQUIRRELS OF ASIA AND AFRICA 237 



the whole skeleton of the fore-limbs be so modified as to 

 form a wing, as in bats, it is difficult to see how ordinary 

 mammals could be endowed with the power of taking flying 

 leaps save by the development of an expanse of skin along 

 the sides of the body in the manner which obtains in the 

 true flying-squirrels, the scale-tailed flying-squirrels, the 

 flying-phalangers, and, it may be added, the flying-lemurs. 



The development of flying membranes in all these four 

 groups of mammals has, in fact, taken place quite inde- 

 pendently, and affords an interesting example of what is 

 known as parallelism in development. Such parallelisms 

 are due, so to speak, to the poverty of possibilities in 

 the way of modification of animal structures. As already 

 said, the simplest and most obvious way of endowing an 

 ordinary four-limbed mammal with the power of taking 

 flying leaps is by the development of lateral expansions 

 of skin. Similarly, the only easily conceivable method by 

 which a primitive short-limbed and many-toed hoofed 

 mammal could be converted into one cut out for speed, 

 like a horse or a gazelle, is by reducing the number of 

 the digits and increasing the length of the lower segments 

 of the limbs. Accordingly, we find parallelism in this 

 respect between the horses and the zebras on the one hand, 

 and the gazelles, antelopes, and deer on the other. 



But the parallelism is by no means exact in this latter 

 case, as indeed would be naturally expected if the lines 

 of evolution were distinct ; and the structure of the lower 

 portion of the limb of a horse differs essentially from the 

 same part in a gazelle. 



Neither is the parallelism exact in the case of the two 

 groups of flying-squirrels. In the flying-squirrels of Europe 

 and Asia, such as the one depicted in the plate, the 

 flying membrane, or parachute, is merely a lateral expansion 



