166 DEVELOPMENT AND HEREDITY. 



change in the fore quarters of the giraffe produce 

 a peculiar relation to the hind quarters, whereby 

 the gallop of the animal is rendered unlike that of 

 all other hoofed animals, as Mr. Herbert Spencer 

 has pointed out. ^ The unity of action, however, 

 is still preserved, and the parts, though changed 

 by their mutual interaction, still work harmoniously 

 together. The changes by which the hinder parts 

 have adjusted themselves to the gradual changes 

 of the fore quarters, we can only attribute to the 

 change in the forces which the two sets of organs 

 bring to bear on each other. These secondary 

 changes, due to the mutual action of parts on each 

 other, embrace a large class, of phenomena, and 

 probably the effects thus produced cannot always 

 be distinguished from those effects directly due 

 to primary external causes. We see from this 

 example that a comparatively slight change in the 

 habit of an animal may, in the course of generations, 

 produce a profound change in its growth, affecting 

 every organ. 



From what has been said of the nature of the 

 hereditary impulse and of the action of the forces 

 which produce the growth of organisms, it follows 

 that any alteration of the forces, in order to produce 

 the greatest change in the organism, must act upon 



1 The Factors of Evolution, 1885. 



