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 offspring 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. 
