274 TH E EVOLUTION OF MAN. 



accidental coincidences, will remain a stranger to the results of this 

 investigation ; not merely because he does not comprehend the conclu- 

 sions, but principally because the significance of the facts on which they 

 are grounded, escapes him. A fact in itself is no more a scientific result, 

 than a mere collection of facts is a science. That which makes a science 

 of these facts, is their combination by that organizing mental faculty which 

 determines the relations of the facts to each other." KAKL GKUKXBAUK 

 (1872). 



AMONG those features of the organization which are specially 

 characteristic of the vertebrate tribe as such, the peculiar 

 arrangement of the motive apparatus, or " locomotorium," 

 undoubtedly occupies a principal place. As in all the 

 higher animals, the active organs of motion, the muscles, 

 form the most important part of this apparatus ; these are 

 the fleshy bands which, by means of their peculiar contrac- 

 tibility, of their power of contracting and shortening, move 

 the various parts of the body, and thus change the position 

 of the entire body. The arrangement of these muscles is, 

 however, entirely peculiar in Vertebrates, and differs from 

 the arrangement common to all Invertebrates. 



In most lower animals, especially in Worms, we find 

 that the muscles form a simple, thin flesh -layer immediately 

 below the outer skin-covering. This " skin-muscle pouch " 

 is most intimately connected with the skin itself, and the 

 same feature occurs in the tribe of the Soft-bodied Animals 

 (MollusccC). In the great group of the Articulated Animals 

 1 'Arthropoda), in the Crab, Spider, Centipede, and Insect 

 classes, we also find a similar feature, but with the difference 

 that in these the skin-covering forms a hard coat of mail ; 

 an inflexible skin-skeleton, formed of chitine, and often of 

 carbonated chalk. This outer chitinous coat of mail is 

 jointed in a great variety of ways both on the trunk and 



