226 GEOLOGICAL BIOLOGY. 



the phylogenetic line of their evolution, and relatively in 

 each line those expressing greater differentiation in the gen- 

 eral development, or higher specialization of the more de- 

 pendent or secondary characters, are necessarily of higher 

 rank, on the theory of acquirement of characters by direct 

 descent only. 



The Principle of Cephalization. — The relative development 

 of the organs of correlation, especially of the organs of sense, 

 has been recognized for many years as indicative of grade of 

 rank among animals. 



James D. Dana has written much on this subject under 

 the name of Cephalization. 



In discussing the principle of Cephalization Dana wrote: 

 "Such growth or progress in the brain and nervous system, 

 the seat of power in the animal, is accordant with, and conse- 

 quent upon, the great fact that this is the part of the struc- 

 ture which comes into actual contact with outside and inside 

 nature. It is the means in the animal by which communica- 

 tion is had with the outer world, and also with its own inner 

 workings and appetites; that which takes impression, which 

 feels whatever inspires energy, prompts to action, exhilarates, 

 or exalts; the part, therefore, which must grow whenever 

 circumstances favor progress, and, at the same time, fail to 

 grow or dwindle under unfavorable circumstances ; which 

 communicates whatever it receives to the being to which it 

 belongs, and in each case to the part or parts responding 

 to its condition ; which reaches every part of the system and 

 dominates in all action and growth, and hence must cause an 

 expression of its own condition in some way on the structure ; 

 which, moreover, must ordinarily produce correlative changes in 

 correlative parts, if any, because in its owai nature and distribu- 

 tion the system of correlation has a full expression " 



" We may, therefore, believe that in all progress in grade, 

 upward or downward, there was involved some change in the 

 animal structure of the kindexpressingdegree of cephalization." 



"Whatever the types of structure in course of develop- 

 ment, there was also a general subordination in the changes 

 to the principle of cephalization." * 



* A, J, Sci., ser. in., vol. xii., Oct. 1876, pp. 245-251. 



