APPENDIX. 79 



Nature for March 14, 1895, this author enunciates 

 twenty theses touching this subject, many of which 

 appear to me apposite and correct, particularly the 

 following: In every species there is a mean specific 

 form round which the variations are symmetrically 

 grouped like shots around the bull's eye of a target. 

 As soon as natural selection comes into play and 

 favors one of these variations it must shift the centre 

 of density. Variations arise by a change in the out- 

 ward conditions of life and can be useful or indiffer- 

 ent; only in the first case will natural selection obtain 

 control of them and "the new variation will get the 

 upper hand and the centre of density will be shifted." 

 This is not germinal selection, but it is the same as 

 what I have referred to in this and in the preceding 

 essay as displacement of the zero-point of variation. 

 Thiselton-Dyer did not draw the conclusion that a 

 definitely directed variation answering to utility re- 

 sulted from this process, which variation alone must 

 cause the disappearance of useless parts, for the reason 

 that he never attempted to penetrate to the causes of 

 the shifting of the zero-point of variation. Neither 

 Fritz Miiller, whose utterances Thiselton-Dyer was ob- 

 viously ignorant of, nor Thiselton-Dyer himself pushed 

 his inquiries beyond the thought that the shifting in 

 question resulted entirely in consequence of personal 

 selection. There is no gainsaying that the degenera- 

 tion of useless organs cannot be explained by personal 

 selection alone, seeing that though the minus varia- 

 tions may possibly have a selective value at the be- 

 ginning of a degenerative process, they certainly can- 

 not have such in the subsequent course of the same, 

 when the organ has dwindled down to a really mini- 

 mal mass of substance as compared with the whole 



