INTERNAL CAUSES OF VARIATION 203 



in favorite directions ? Does it vary progressively because 

 impelled in these directions by "growth force" or other inherent 

 energy ? Are species held to their present standards by outside 

 influences ? or, if not " held," are they drifting in spite of us ? 

 Is the life principle constant or periodic in its activities ; and 

 are those internal energies that vitalize matter and that determine 

 development and differentiation, are they indifferent as to the 

 trend of the type, or do they run more easily in some channels 

 than in others ? Is variation in some sense subject to and directed 

 by a natural bias ? This is the field of bathmic evolution, 1 and 

 these are the questions involved. No one is more interested in 

 their discussion than is the breeder of domesticated forms. 



Two principal theories covering the field of bathmic evolution 

 have been proposed, both incapable of absolute proof, as all such 

 theories must be, but both of interest to the breeder. 



Acceleration or retardation of growth force. This principle is 

 announced by Cope 2 as an internal and ever-present cause of 

 progressive evolution, running through all forms of life and 

 beneath all ordinary influences, impelling unnoticeably but irre- 

 sistibly in certain directions. It is, after all, according to this 

 author, the most subtle and most potent cause of departure from 

 type. The horse has undergone steady progressive development 

 from an animal of the size of a jack rabbit up to his present 

 proportions and perfection. This is due, according to Cope, 

 not so much to selection as to a continuous, perhaps almost 

 unprecedented, acceleration of growth force. 



This theory attempts to explain much of evolution through 

 the energy of growth, thus throwing into the discussion a 

 dynamic element commonly neglected by evolutionists. In this 

 connection Pearson pertinently remarks : 



There is nothing more (or less) unscientific in using an inherent growth 

 force to explain the secular changes in living forms than in using the force 

 of gravitation inherent in matter to explain the development of planetary 



1 Pearson, Grammar of Science, pp. 375-377. The term "bathmic" as here 

 used does not include genetic selection or any other selective agent, internal or 

 external, because the effects of all such influences tend to come to a rest and 

 not to constitute a "continual bias." 



2 Cope, Origin of the Fittest, pp. 18-30, 190-192, 396-398; Primary Factors 

 of Organic Evolution, pp. 473-494. 



