194 PRIMITIVE FORMS OF LIFE 



to the trunk of the tree. In Urofilatus of Madagascar l the 

 skin runs back along the whole length of the head, trunk, 

 limbs, and tail, which is flattened into the shape of a trowel. 

 An analogous bordering of skin spreads out considerably in 

 Ptychozoon of the Malay Archipelago, 2 and extends web-fashion 

 between the digits of the feet. This brings us to the remarkable 

 case of the Flying-Dragon of the Sunda Islands, in which the 

 skin at the sides forms a kind of a parachute supported by 

 bony rays attached to the ribs. It is probable that analogous 

 conditions gave rise to the wing of the Pterodactyls and 

 the other Pterosaurians of the Secondary Period. This wing 

 resembles that of the Bat in its mode of formation, but 

 instead of being supported by the four outer digits, the 

 pollex remaining free, as with the Bat, it is supported only 

 along the length of its anterior border by a single outer 

 digit which is greatly elongated. 



Though it may be true that in certain cases we can explain 

 the intimate adaptation of animal organs to the functions they 

 carried out by supposing that these organs were formed with 

 no particular end in view, and that those animals, thus enabled 

 to lead a certain kind of life forbidden to others not thus 

 provided, profited by these organs to live an existence for which 

 they found themselves in some measure fire-adapted, the facts 

 we have just been enumerating show us clearly enough that 

 this hypothetical fire-adaptation can give us only an incomplete 

 view of the truth. Moreover, the word pre-adaptation itself 

 suggests the notion that animals have been formed in advance 

 to live in a predestined manner, and comes dangerously near 

 to reviving the old doctrine of determinism. 



By the very fact that it is alive, an organism cannot be 

 considered to be passive. If it is subject to the influence of light, 

 heat, humidity, dryness, the regular return of day and night, 

 the periodicity of the seasons, in a word to everything that is 

 called external environment ; this influence must also react 

 profoundly on its internal environment, which thus becomes 

 a powerful agent of modification. Every living cell by the very 

 fact that it feeds, every muscular element that contracts, every 

 gland cell that secretes, and every neuron that undergoes 

 or elaborates a stimulus, pours into this interior environment 

 some substance capable of acting upon the cells with which it 



1 LXI, 259. 2 LXII, 512. 



