130 PRIMITIVE FORMS OF LIFE 



the parts of the body directly subjected to their influence and 

 not affecting other parts ; which is equivalent to saying that 

 these external actions are the causes of the modifications which 

 are correlated with them. This correlation is found in an 

 infinite number of cases. The legs of the swimming Arthropods, 

 for instance, which move them about only by thrusting 

 against the water, are flattened and acquire a fringe of 

 long hairs ; a web appears between the toes of the foot in all 

 walking Vertebrates, that return to the water, whatever the 

 type to which they belong : tailless batrachians, crocodiles, 

 pond and marsh tortoises, web-footed birds, Omithorhynchus, 

 desman, musk-rat, beaver, mink, otters, seals, etc. In all the 

 groups of climbing vertebrates which press their abdomen 

 against the trunks of trees, we likewise find species that 

 have lateral skin-folds running down on to their limbs in such 

 a way as to reach as far as the digits. This condition is 

 reproduced in Petaurus, among the Marsupials, in the lemur 

 Microcebes, in the insectivorous Galiopithicus, Pteromys 

 sibericus, and in the squirrel Anomaturus ; and in reptiles 

 that lead up to the flying dragons, such as Stychozoon and 

 Uroplatus. This condition prepared the way for the wings of 

 the Pterodactyles or Flying Reptiles of the Secondary Epoch, 

 and for mammals like the bats. 



Certainly the kind of life led does not suffice to bring about 

 this transformation. Otherwise all climbing animals, for 

 example, would have acquired parachutes. Certain organic 

 conditions are also required which we as yet cannot define, 

 or perhaps a minimum degree of frequency in the repetition of 

 the same acts permiting the modifying forces to assert them- 

 selves with especial intensity, or the regular co-operation 

 maybe of many of these forces acting simultaneously. At all 

 events, the correlation is too frequent for us to believe that it 

 is independent of a causal factor. 



An explanation of this correlation has been attempted by 

 appealing to what have been called pre-adaptations. New 

 characters make their appearance without our being able to 

 assign a reason ; they would be cryptogerous, as geologists 

 say of species that suddenly make their appearance in 

 certain geological layers, without it being possible to discover 

 for the moment whence the} r have come. Throw-backs due to 

 heredity, unknown modifications of the internal environment, 



