CAUSES OF EVOLUTION 



249 



the individual's metabolic processes as to give rise to modifica- 

 tions of the constitutions of certain corresponding proteins and 

 other vital molecules which, even though they be of too subtle 

 a character for the chemist to detect by his present methods, 

 may nevertheless be sufficient to cause not only physiological 

 and morphological differentiations in the individual, but also 



Fig. 118. Short-Fingeredness (Brachydactyly) and Long-Fingeredness (Dolicho- 

 DACTYLv). Congenital, and Due to Internal Secretion. 



(Left.) Congenital brachydactyly, theoretically due either to a sudden alteration in the 

 chromatin or to a congenital defect in the pituitary gland. After Drinkwater. 



(Centre.) Brachydactyly, after birth, due to abnormally excessive secretions of the 

 pituitary gland. After Gushing. 



(Right.) Dolichodactyly, after birth, due to abnormally insufficient secretions of the 

 pituitary gland. After Gushing. ' 



become manifested physiologically [functionally] and morpho- 

 logically [structurally] in the offspring." 



The above summary adumbrates the lines along which some 

 of the chemical interactions, if not causes, of mammalian ev- 

 olution may be investigated during the present century. 



The cause of different bodily proportions, such as the very 

 long neck of the tree-top browsing giraffe, is one of the classic 

 problems of adaptation. In the early part of the nineteenth 

 century Lamarck (p. 143) attributed the lengthening of the neck 



