208 CAUSES OF VARIATION 



careful student of evolution doubts any longer that there are 

 many misfits in nature. Whether they have arisen, as Eimer 

 asserts, by reason of organic growth, and whether they are evi- 

 dences of definitely directed deviation, is quite another matter. 



There is no doubt of the persistence of a character once started, 

 even in the face of selection, but whether it be necessary to 

 invoke the aid of an internal directive force to explain it is a 

 question upon which more evidence is sorely needed. It is 

 exceedingly important for the breeder to know and recognize all 

 the inherent tendencies with which he must finally reckon, and 

 it may be necessary to go beyond physiological units, correspond- 

 ing to chemical atoms or molecules, and invoke some form of 

 "growth force " corresponding to chemical energy to explain the 

 mysteries of development. Any theory, however, that will even 

 reasonably account for these mysteries must be, in the present 

 state of knowledge, largely an assumption, and let the assump- 

 tion be as simple as possible until we can defend its complexity 

 by a mass of well-established facts. In the opinion of the writer 

 the existence of such a principle as orthogenesis is more than 

 problematical, except as it is an expression of the relations 

 that naturally obtain between physiological units, whatever they 

 may be. 



SECTION XV PHYSIOLOGICAL UNITS 



The "gemmules" of Darwin, the " stirp " of Galton, the 

 "idioplasm" of Nageli, the "biophors," "determinants," and 

 "ids" of Weismann, and the " physiological units" of other 

 writers are all attempts to explain inheritance of definite quali- 

 ties by assuming that the germ cell which passes over from 

 parent. to offspring at the time of procreation is composed of 

 definite units of living matter, each with its specific properties, 

 among which are nutrition and multiplication, which together 

 constitute growth, and considering the separate properties of 

 the different units of which a given individual is composed 

 growth in definite directions. 



In support of this general theory it may be urged that the indi- 

 vidual is what he is very largely because of internal qualities. 

 Corn and wheat grow side by side, drawing their nourishment 



