OTHER THEORIES OF SPECIES-FORMING 265 



WEISMANN'S THEORY OF GERMINAL SELECTION 



This theory was intended to rehabilitate the selection principle 

 which had lost a great deal of prestige because of the serious character 

 of the objections that had been raised against it, most of which have 

 been stated in the last chapter. The theory is believed by its author 

 to overcome all objections and doubts and to clear away all difficulties. 

 "Its strength," says Plate, "shall avail in four directions. First, it 

 shall explain how not only degeneration (physiological) but rudimenta- 

 tion (morphological) occurs in panmixia; second, why exactly those 

 variations needed for the development of a certain adaptation appear 

 at the right time; third, how correlation of adaptation comes to exist; 

 and fourth, how variations are able to develop orthogenetically along 

 a definite line without depending on the necessity of a personal selec- 

 tion raising them step by step." 



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 



