612 Validity of the Doctrine of Mutation. 



IV. TJie theory of selection explains the existence of 

 useful characters, but does not explain that of useless or 

 actually harmful ones.^ Whereas the doctrine of muta- 

 tion assumes that specific differentiation does not take 

 place in any definite direction, that mutations are pro- 

 duced independently of their adaptive value, and that they 

 may survive, provided that they do not prejudice the 

 existence or annul the fertility of the individual, the 

 theory of selection cannot account for the origin of sterile 

 forms of wdiich, nevertheless, there are many. I have 

 already mentioned instances of these in the first part of 

 this volume, and will here only add a reference to a most 

 remarkable sterile form of oats, recently described and 

 figured by Noll ;- to the dates'^ and grapes'* without pips, 

 and to the highly branched and absolutely sterile variety 

 of our gardens, called Muscari comosur.i pluniosuni, 

 wdiich belongs to a species characterized by a tuft of 

 sterile flowers at the tip of the normal spike (Fig. 136). 



Instead of giving a further discussion I shall content 

 myself with referring to the curious case of Mimidiis 

 and Torenia, which Burck has described, and which this 



theory of selection is most evidently insufficient. See Wasmann. 

 Biol. Centralhlatt, Vol. XXI, Nos. 22, 23; also Emery, Gedankcn zur 

 Desccndeua- iind V ererhiingstheovic , Biol. Centralhlatt, 1893. Vol. 

 XIII, Nos. 13 and 14, p. 397; further W. Wagner, Vindusfric dcs 

 Arancina, Mem. Acad. Imp. St. Petershourg, Vllth Ser., Vol. XLII, 

 No. II, 1894; and N. Cholodkovsky^ Die Coniferenldiise, Hor. Soc. 

 Ent. Ross., XXXI, p. 43. 



' For a list of these T refer the reader to Demoor, Massart and 

 Van der Velde, L'cvohition regressive, Paris, 1897, especially pp. 

 286-289. 



^F. Noel, Sifaungsber. d. Niederrhein. Gcs. f. Nafiirk., Bonn, 

 March 4, 1901. 



^ Ch. Riviere, Societe nat. d'accUmatation, Paris, La Nature, 

 1901, No. 1477, p. 247. The tree in question grow^s near Hamma in 

 Algeria. 



*H. Muller-Thurgau has given an exhaustive list of wholly 

 or partially sterile varieties of grapes in Experiment Station Record, 

 XT, p. 16, 1902. 



