28 STUDIES IN EVOLUTION 



thrust out in more or less the same direction. The presence 

 of a favorable exciting cause, like a particle of food, produces 

 extension of the protoplasm to envelop it. 



Furthermore, as is well known, continuous extra-pressure 

 on any part of an organism produces atrophy and absorption, 

 and intermittent or occasional pressure causes hypertrophy 

 and growth. That the pressure should be intermittent seems 

 a necessary condition for hypertrophy, in order that the 

 parts affected may have normal intervals allowing the active 

 exercise of nutrition.^^ This may be regarded as a parallel 

 statement of the law of disuse and use ; the former causing 

 organs or parts to dwindle away and lose their function, and 

 the latter producing increased nutrition and growth. 



This ratio of exchange between nutrition and waste is on 

 the side of full or excessive cell-nutrition, producing growth 

 in the parts affected, while deficiency of nutrition produces 

 decline or suppression. If the successive increment constitut- 

 ing growth is along definite progressive lines towards higher 

 structures, and the decrement affects the decline of useless 

 parts or permits of the replacement of a lower by a higher 

 structure, then the sum of the changes is progressive 

 evolution.* 



Growth, as stated, seems to require normal intervals for 

 the proper exercise of nutrition, which involves an inter- 

 mittence of the exciting or stimulating forces. Rhythm has 

 Been shown by Spencer®® to be a necessary characteristic of 

 all motion, and therefore in considering either the intrinsic 

 or extrinsic forces acting on the structures of an organism, 

 they must be rliythmic 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 intermittent, and their resultant must have a 

 * Tliis is very near Cope's idea of progressive evolution. 



