ORGANIC EVOLUTION 291 



The theory of panmixia attempts to explain how organs degenerate, 

 which natural selection cannot explain unless the abandoned organs are 

 injurious. Natural selection is assumed to select favorable structures 

 and make them still more favorable, but not to eliminate structures 

 that have simply become useless. According to Weismann, when selec- 

 tion ceases to operate upon a certain organ because it has become useless 

 under new conditions, individuals with this organ poorly developed will 

 no longer be at a disadvantage and therefore will survive. The crossing 

 of individuals with this organ in all stages of effectiveness will result in 

 the next generation in lowering the general level of efficiency, and the 

 organ as a whole will appear degenerate. This general mixing, which 

 lowers the average of efficiency, is called panmixia. It is impossible to 

 explain, however, how panmixia could lead to a continuous degeneration 

 of the organ involved. 



Weismann's theory of germinal selection (1895) is one of the most in- 

 genious speculative explanations of the beginnings of variation and of 

 determinate variation (orthogenesis) that has been proposed, neither of 

 which natural selection seemed able to explain, for it can operate only 

 upon variations that have been carried forward to the point of distinct 

 advantage, and it cannot carry forward a variation in spite of changing 

 conditions. Weismann differentiated between somatic protoplasts, 

 which give rise only to the vegetative cells of the plant or animal body, 

 and germ protoplasts (" germ-plasm "), which give rise to the reproduc- 

 tive cells. The nuclei of the protoplasts contain large numbers of 

 imaginary living units (biophores), and these units are organized into 

 groups (determinants) which determine the character of the cell. Each 

 kind of somatic cell is supposed to be produced by a certain kind of 

 determinant ; but a germ cell contains all the determinants that belong 

 to all the cells of the body. The structure of the ofifspring depends 

 upon the determinants that are favored in development, and this at first 

 seems to be a matter of chance in food supply. There results a 

 " struggle " among determinants, and a " germinal selection." The 

 stronger determinants that become established in the germ-plasm, how- 

 ever, are handed down generation after generation, and therefore a 

 variation once begun may continue until it can be laid hold of by nat- 

 ural selection, or may even continue as the persistent determinate varia- 

 tion recognized by orthogenesis. 



Ingenious as this explanation is, it must be stated that it rests upon no 

 demonstration, and that there are serious objections to it. 



