On the Mechanical Conception of Nature. 665 



never been observed to a perceptible extent in 

 other species.* 



All variations which are demonstrably useful 

 can be similarly dealt with if their origin is ex- 

 plained by variational tendencies. 



We perceive that the objection which Von 

 Hartmann brings against heredity is only valid on 

 the ground that this process affords no security 

 for the preservation of variations which occur 

 singly. That heredity itself is a mechanical pro- 

 cess is not directly disputed ; it is simply assumed 

 that new characters can be transferred by in- 

 heritance only when they are produced by the 

 metaphysical " developmental principle," and not 

 when they arise " accidentally." This critic does 

 not therefore direct his attack against heredity, 

 but rather against the mechanical origin of varia- 

 bility. 



Von Hartmann might have said here that a 

 reference of the phenomenon of heredity to purely 

 mechanical causes, i. e. a mechanical theory of 



[A certain number of instances of mimicry are known to 

 occur between species both of which are apparently nauseous. 

 A most able discussion of this difficult problem is given by 

 Fritz Miiller, in the case of the two butterflies Ituna Ilione 

 and Thyridia Megisto, in a paper published in Kosmos> May, 

 1879 (p. 100). The author shows by mathematical reasoning 

 that such resemblances between protected species can be 

 accounted for by natural selection if we suppose that young 

 birds and other insect persecutors have to learn by experience 

 which species are distasteful and which can be safely de- 

 voured. See also Proc. Ent. Soc. 1879, pp. xx xxix. R. M.] 



