294 THE INDUCTIONS OP BIOLOGY. 



is associated with a decrease in the excess of tissue-producing 

 power. We cannot say, indeed, that this decrease 



always results in gamogenesis : some organisms multiply for 

 an indefinite period by agamogenesis only. The Weeping 

 Willow, which has been propagated throughout Europe, does 

 not seed in Europe; and yet, as the Weeping Willow, by its 

 large size and the multiplication of generation upon genera- 

 tion of lateral axes, presents the same causes of local innutri- 

 tion as other trees, we cannot ascribe the absence of sexual 

 axes to the continued predominance of nutrition. Among ani- 

 mals, too, the anomalous case of the Tineidce, a group of moths 

 in which parthenogenetic multiplication goes on for genera- 

 tion after generation, seems to imply that gamogenesis does 

 not necessarily result from an approximate balance of assimi- 

 lation 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 gamogenesis; but that there appear to be 

 other conditions, in the absence of which approach to equili- 

 brium 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 

 recur? — 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 certain 

 hypothetical answer. 



Seeing, on the one hand, that gamogenesis recurs only in 

 individuals which are approaching a state of organic equili- 

 brium; and seeing, on the other hand, that the sperm-cells 

 and germ-cells thrown off by such individuals are cells in 

 which developmental changes have ended in quiescence, but 

 in which, after their union, there arises a process of active 

 cell-formation; we may suspect that the approach towards a 



