470 THE EVOLUTION OF LIFE. 



will naturally result these communities of fundamental 

 structure among creatures which have severally become 

 modified in multitudinous ways and degrees, in adaptation to 

 their respective modes of life. To this let it be 



added that while the belief in an intentional adhesion to a 

 pre-determined pattern throughout a whole group, is nega- 

 tived by the occurrence of occasional deviations from the 

 pattern; such deviations are reconcilable with the belief in 

 evolution. As pointed out in the last chapter, ancestral 

 traits will be obscured more or less according as the super- 

 posed modifications of structure, have or have not been fur- 

 thered by the conditions of life and development to which 

 the type has been subjected. 



§ 134. Besides these wide-embracing and often deeply- 

 hidden homologies, which hold together different animals, 

 there are the scarcely-less significant homologies between dif- 

 ferent organs of the same animal. These, like the others, 

 are obstacles to the supernatural interpretations and sup- 

 ports of the natural interpretation. 



One of the most familiar and instructive examples is 

 furnished by the vertebral column. Snakes, which move 

 sinuously through and over plants and stones, obviously need 

 a segmentation of the bony axis from end to end; and inas- 

 much as flexibility is required throughout the whole length 

 of the body, there is advantage in the comparative uniformity 

 of this segmentation. The movements would be impeded if, 

 instead of a chain of vertebrae varying but little in their 

 lengths, there existed in the middle of the series some long 

 bony mass that would not bend. But in the higher Verte- 

 hrata, the mechanical actions and reactions demand that 

 while some parts of the vertebral column shall be flexible, 

 other parts shall be inflexible. Inflexibility is specially requi- 

 site in that part of it called the sacrum ; which, in mammals 

 and birds, forms a fulcrum exposed to the greatest strains 

 the skeleton has to bear. Now in both mammals and birds. 



