OH the Mechanical Conception of Nature. 669 



formula, scarcely furnishes any deeper knowledge. 

 In the light of a theory, detailed instances which 

 were formerly obscure should become compre- 

 hensible. I fail to see, however, how the 

 various forms of atavism, e.g. the reversions which 

 so commonly occur by crossing different races, 

 become more comprehensible by assuming con- 

 sciousness in the plastidiile. If in both parents 

 the plastidule long ago acquired different molecu- 

 lar motions, why, in its rencounters in the germ, 

 does it recollect past times and reassume the 

 older and long abandoned motion ? That it does 

 acquire the latter is indeed a fact if we once refer 

 the directional development of the individual to 

 molecular motion of the plastidule ; the wherefore 

 does not appear to me, however, to become 

 clearer by assuming consciousness in the plasti- 

 dule. A mechsnical theory of heredity must 

 rather be able to show that the plastidule move- 

 ments of the male and female germ-cells, in their 

 rencounter in the case of the crossing of widely 

 divergent forms, become mutually modified in 

 such a manner that the motion of the common 

 ancestral form must occur as the resultant. To 

 such demonstration there is however as yet a long 

 step. Haeckel himself moreover points out that 

 his hypothesis is by no means a " mechanical 

 theory of heredity," but only an introduction to 

 this theory, which he hopes " will be capable of 

 being elevated to the rank of a genetic molecular 



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