CHANGE FACTORS. INTRODUCTION 309 



creased functioning, would give off pangenes reflecting muscle or eye 

 improvements. Similarly, a deteriorating organ or tissue or even a 

 damaged or diseased part would produce pangenes reflecting their con- 

 dition. Now these pangenes were believed to be collected in the gonads 

 to form germ cells, a germ cell being a concentrated mass of all the 

 kinds of pangenes of the organism. In this simple way somatic changes 

 were believed to be inherited. But the difficulty with this theory is 

 that no such mechanism exists. 



Weismann was the first biologist to depart from the idea that 

 hereditary changes originate in the soma. He proposed the theory of 

 the continuity and apartness of the germ plasm, and its corollary that 

 all hereditary changes arise directly in the germ plasm. His picture 

 of the method of germinal change is expressed in his theory of "germi- 

 nal selection." 



The essential feature of germinal selection, as the name implies, 

 is a transfer of the struggle for existence to the germ cell. The germ 

 cell is assumed to be a greatly reduced and simplified sample of the 

 characters of the whole organism. Each independently variable part 

 of the organism is supposed to be represented in the germ cell by a 

 minute physiological unit, unique in composition and capable of 

 reproducing the part in question in a new organism. These hereditary 

 units are called "determinants." Thus there is a different kind of 

 determinant for each muscle of the body, for each bone, or for each 

 independently functioning blood vessel; but, since all red blood cor- 

 puscles are alike, there would be only one determinant for all of them. 

 These determinants have to grow, and in cell division, to divide so as to 

 furnish to daughter germ cells all of the necessary determinants for a 

 whole individual. In their process of growth and multiplication, 

 which goes on very rapidly at certain periods in the germ-cell cycle, 

 these determinants are in competition among themselves for the 

 available food supply. Some may be more favorably placed than 

 others or may be more active chemically than others. There will thus 

 arise a struggle within the germ for a chance to grow and reproduce 

 their kind, which, for these determinants, might be as bitter as would 

 be the struggle in nature among the whole organisms that are in com- 

 petition for a place in the world. A determinant favored, perhaps 

 accidentally or possibly because of inherent activity, by a good food 

 supply would wax stronger and grow faster and would, logically, pro- 

 duce a larger and more effective part when that particular germ cell 

 developed into an adult. Other germ cells that would be the offspring 



