GENESIS. 233 



partially and wholly discrete organisms, like most plants and 

 many inferior animals, this approach to equilibrium results 

 from absolute or relative decline of nutrition ; matters not. 

 In any case, the recurrence of gambgenesis is associated with 

 a more or less marked decrease in the excess of tissue-pro- 

 ducing power. We cannot say, indeed, that a de- 



crease in this excess always results in gamogenesis ; for we 

 have evidence to the contrary, in the fact that some organ- 

 isms multiply for an indefinite period by agamogenesis only. 

 Thus, the weeping willow, which has been propagated through- 

 out Europe, does not seed in Europe ; and yet, as the weep- 

 ing willow, by its large size and the multiplication of 

 generation upon generation of lateral axes, presents the same 

 causes of local innutrition as other trees, we cannot ascribe 

 the absence of sexual axes to the continued predominance of 

 nutrition. Among animals, too, the anomalous case of the 

 Tiiieidce, a group of moths in which parthenogenetic mul- 

 tiplication goes on for generation after generation, shows us 

 that gamogenesis does not necessarily result from an approxi- 

 mate balance of assimilation by expenditure. What we must 

 say, is, that an approach towards equilibrium between the 

 forces which cause growth and the forces which oppose 

 growth, is the chief condition to the recurrence of gamo- 

 genesis ; but that there are other unknown conditions, in the 

 absence of which this approach to equilibrium is not followed 

 by gamogenesis. 



79. The above induction is an approximate answer to 

 the question When does gamogenesis recur ? but not to the 

 question which was propounded Why does gamogenesis re- 

 cur ?--Why cannot multiplication be carried on in all cases, 

 as it is in many cases, by agamogenesis ? As already said, 

 biologic science is not yet advanced enough to reply. Mean- 

 while, the evidence above brought together, suggests a cer- 

 tain hypothetical answer, which it may be well to set down. 

 Seeing as we do, on the one hand, that gamogenesis recur a 



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