C. E. Beecher — Origin and Significance of Spines. 127 



extrinsic forces acting on the structures of an organism, they 

 must be rhythmic or intermittent. In the environment, the 

 most apparent changes are those of light and darkness, heat and 

 cold, moisture and dryness, and variations in amount of oxygen, 

 all of which affect an organism directly, and also through the 

 accompanying variations in the character and amount of the 

 food supply, the number of enemies, etc. These and most of 

 the mechanical forces of the environment are therefore inter- 

 mittent, and their resultant must have a 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 immediate 

 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. 10 The internal energy of growth, 

 involving the capacity or effort of responding to external 

 stimuli, is termed entergogenic energy by Hyatt. 34 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." 34 

 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 principally congenital or phylogenic. If, on the other 

 hand, the external forces predominate, the modifications will 

 be principally adaptive, or ontogenic. In each case, the 

 resultant is the actual visible effect of the two. If both are 

 toward the establishment 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 devel- 

 opmental variations occurring in the life history of any great 

 group of organisms. Anyone who has studied the chronologi- 

 cal 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 found at an early period, 

 while the greatest specific differentiation occurs at a later 

 period. This shows that the results of variation at first affect the 



