ORIGIN AND SIGNIFICANCE OF SPINES 29 



definite tendency, so that the effects are not with each change 

 successively positive and negative to the same degree ; that 

 is, the same structures or adjustments are not alternately 

 made and unmade. 



It is generally recognized that there is a necessity for a 

 force or energy in living organisms, which is not the imme- 

 diate and direct result of external agencies, but upon which 

 these fall and produce reactions. It is considered as a phase 

 or kind of vital force directing growth, and therefore a 

 growth force, or the bathmic force of Cope.^^ The internal 

 energy of growth, involving the capacity or effort of respond- 

 ing to external stimuli, is termed entergogenic energy by 

 Hyatt. ^* Without this power an organism would be unable 

 to move or respond to external stimuli. The effect of the 

 action of this kind of energy must be the resultant between 

 "the structures already existent in the organism and the 

 external forces themselves."^* Since the growth force is 

 within the organism, or inborn, it is one of the principal 

 characters transmitted through heredity, and if it is in excess 

 of the external forces, the modifications will be princijDally 

 congenital or phylogenic. If, on the other hand, the external 

 forces predominate, the modifications will be principally adap- 

 tive, or ontogenic. In each case the resultant is the actual 

 visible effect of the two. If both are toward the establish- 

 ment of similar structures, their effect will be the sum of the 

 two; but if they are opposed to each other, the effect will 

 be their resultant, the nature of which, as seen above, will 

 depend upon their relative power. 



These conclusions can be correlated directly with the 

 developmental variations occurring in the life history of any 

 great group of organisms. Any one who has studied the 

 chronological development or the phylogeny of a class of 

 forms cannot fail to have been impressed with the fact that 

 all types of life are physiologically more plastic or subject 

 to greater changes near their point of origin. That is, the 

 maximum of generic, family, and ordinal differentiation is 



