PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 527 



These differences between the mouth, jaws, and gills of verte- 

 bates and those of the arachnids are as significant, therefore, as the 

 resemblances, and point to the same conclusion. For the peculiar 

 conditions found in the vertebrates are seen to be the inevitable result 

 of the conditions prevalent in the arachnids ; here, as elsewhere, 

 growth inevitably occupies the easiest paths of conveyance, and the 

 products accumulate along the paths of least resistance. 



Fig. 9. Diagrams indicating the probable manner in which the arachnid 

 gill sacs, in vertebrates, come to open into the alimentary canal. The usage 

 of these passages for respiration prevents their usage for digestion. The 

 anterior haemal outgrowths, which never communicate with gill sacs, give 

 rise to the thyroid glands. A, scorpion. B, hypothetical transition stage. C, 

 vertebrate. From Patten, " The Evolution of the Vertebrates and their Kin." 



In these particular cases, the forced migration of the jaws to the 

 haemal surface, the formation of a new mouth outside the medullary 

 plate, and the opening of the gill sacs into the alimentary canal, 

 result in mechanical and economic improvements of the greatest im- 



