u- 



t <f rf^ing 

 



40 STUDIES IN EVOLUTION 



unfavorable environment, lack of physiological plasticity, 

 too close interbreeding, pathologic influences, and parasitism. 

 The first commonly implies a scarcity of food, or it may be 

 that the temperature, moisture, light, elevation, or other 

 conditions are unsuitable to the normal development. The 

 lack of physiological plasticity affects growth force by its 

 resistance to change, and is most strongly apparent in highly 

 forms. The effects of close interbreeding in reduc- 

 vitality are too well known to require further notice. 

 Only in exceptional instances can individual pathologic 

 ( Conditions have any effect on a stock. The retrogressive 

 . t/V^- f!$eries of animals which are diseased in appearance, and are 

 jjC^^lf. considered by Hyatt * as akin to pathologic distortions, are 

 apparently types which have ceased to advance physiologi- 

 cally, and are therefore only adapted to special sets of con- 

 ditions. In these, the pathologic or abnormal condition is 

 racial instead of individual, and its cause seems to be a 

 deficiency of vital power combined with great external differ- 

 entiation, the final result being the assumption of characters 

 belonging to second childhood and ending in extreme senility, 

 with the loss of spines and other ornaments. 



The life history of parasitic organisms shows their origin 

 from higher normal types by a process of retrogression through 

 loss of motion and disuse of parts. Their mode of living 

 implies dependence upon the vitality of an immediate host, 

 and altogether they may be deficient in the energy of growth 

 force. 



Any of the preceding factors, single or combined, acting 

 upon an organism or group of organisms will produce sup- 

 pression of structures or functions. Whether from external 

 or internal causes, the waning and disappearance of characters 

 are almost always inversely to the order of their development 

 or appearance, either in the race or in the individual, and 

 the most primitive or axial characters are therefore the most 

 persistent and the last to disappear. In this way a leaf may 

 be suppressed into a spine representing the midrib, a branch 

 into a spiniform twig, a leg or digit into a spine, etc. As 



