OTHER THEORIES OF SPECIES-FORMING. 191 



but how can it cause organs no longer useful and advan- 

 tageous to degenerate? 



It is possible, perhaps, to explain the eradication of 

 positively harmful organs by a process of negative or 

 reversed selection. If an organ becomes actually harm- 

 ful because of a change in life conditions, individuals 

 with the organ in poorest, least energetically functioning 

 condition might be conceived to have an advantage and be 

 preserved by selection to pass on to their offspring this 

 less developed, /. e., rudimentary or vestigial, character of 

 the particular organ in question. But when the organ is 

 simply only rendered useless by the change in life condi- 

 tions, as when a species of fish or insect gradually comes to 

 inhabit deep dark caves and thus has no more use for its 

 eyes, how does selection explain the degeneration ? It really 

 doesn't, satisfactorily. So Weismann offers the theory of 

 panmixia to account for it. This is, simply, that owing to 

 the cessation of selection in regard to the particular organ 

 whose function is rendered no longer advantageous or 

 necessary under the new life conditions that this cessation 

 of selection is an obvious result of such a state of affairs 

 was recognised by Darwin himself, and by other biologists 

 individuals born with this organ defective or in a condition 

 below the average, would not be necessarily killed by the 

 rigours of the intra-specific struggle, and would therefore be 

 as likely to mate and keep on producing offspring as the 

 ones with the organ in average or above average conditions. 

 This general participation of all kinds of individuals (all 

 kinds, that is, as regards the state of the particular organ) 

 in producing the next generation, and the continued repeti- 

 tion of this general mixing, panmixia, would obviously lead 

 to a reduction of the earlier high condition of development 

 of the organ. Weismann thinks, or thought, it would lead 

 to a steady degeneration of the organ. But few other biol- 

 ogists, even those ardent selectionists anxious to find in 



