382 THE EVOLUTION OF T TFK. 



as tlie superposed modifications of structure, have or tave 

 not been great or long maintained. Hence, thougli the occur- 

 rence of articulate animals, such as spiders and mites, having 

 fewer than tvrenty segments, is fatal to the supposition that 

 twenty segments was decided on for the three groups of 

 superior Articiilata; it is not incongruous with the supposition, 

 that some primitive race of articulate animals, bequeathed to 

 these three groups this common typical character — a character 

 which has nevertheless, in many cases, become greatly ob- 

 scured, and in some of the most aberrant orders of these 

 classes, quite lost. 



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

 hidden homologies, which hold together diflPerent animals, 

 there are the scarcely-less significant homologies between 

 different organs of the same animal. These homologies^ 

 like the others, are obstacles to the supernatural interpreta- 

 tions, and supports of the natural interpretation. 



One of the most familiar and instructive instances 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 

 inasmuch as flexibiKty is required throughout the whole 

 length of the body, there is advantage in the comparative 

 uniformity of this segmentation : the creature's 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 most 

 of the higher Vertehrata, the mechanical actions and reac- 

 tions demand that while some parts of the vertebral axis shall 

 be flexible, other parts shall be inflexible. Inflexibility is 

 especially requisite in that part of the vertebral column 

 called the sacrum ; which, in mammals and birds, forms a 

 fulcrum exposed to the greatest strains which the skeleton 

 has to bear. Now in both mammals and birds, this rigid 

 portion of the vertebral column is not made of one long 



