STRUCTURAL MODIFICATIONS 131 



and various external actions whose influence it has not been 

 possible to determine — all of these may give rise to new 

 characters without their being of any immediate utility, but 

 once present, the animal, which at first had only to exercise 

 them, will make use of them as soon as it discovers how 

 to do so. 



Thanks only to these new characters, which appear to have 

 resulted from the new conditions of its existence, the animal 

 was able to profit by the changed conditions. These pre- 

 adaptations are real, and it is due to them that natural selection, 

 which results from the struggle for existence, was so efficacious, 

 and it is they that have given Darwin's theory its whole value. 

 We must not conclude from this that the conditions of existence 

 or of development do not encourage the appearance of new 

 characters in harmony with them, either directly or through 

 their reactions on the animal. Often, on the contrary, pre- 

 adaptations and adaptations overlap. This is what has 

 happened to the Birds. Their feathers were not meant for 

 flying, but were at first merely tegumentary overlapping 

 excrescences, doubtless irregularly branched ; these branches 

 because of the deterioration resulting from the mode of super- 

 position, ended by developing laterally only ; thus those of 

 the wings and the tail were utilizable for support in the air. 

 The foot of the Bird, on the contrary, everywhere bears witness 

 to its will to stand erect on its hind limbs, which it straightens 

 by aid of its muscles to the point of using its toes only as 

 supports. The great toe ends by no longer touching the ground ; 

 it becomes atrophied, but is still represented by the spur of 

 cocks. Those muscles which extend from the pelvis to the 

 thigh and maintain the body erect, having extra work to 

 accomplish, increase in size, and cause considerable growth of 

 the pelvic bones, which invade more and more of the vertebral 

 region both behind and in front of the hip-joint socket. Here 

 we have an evident triumph of the Lamarckian principle of 

 the influence of use and disuse of organs, a principle alien to 

 the origin of the feathers, if not to the determination of their 

 final form. None of these characters were developed with a 

 view to flight ; it was simply a matter of co-ordination of bones 

 and muscles favourable to the biped posture and to hopping, 

 as is proved by the Iguanodons and other herbivorous bird- 

 legged Reptiles of the Secondary Period, or Compsognathus, 



