48 Darwin, and after Darwin. 



they would sooner or later become fused into a 

 common type — supposing, of course, no other form 

 of isolation to be present. The necessity then for 

 this physiological form of isolation in maintaining 

 a specific differentiation which has been already at- 

 tained cannot be disputed. Yet it has been regarded 

 as " Darwinian heresy " to suggest that it can have 

 been of any important service during the process of 

 attainment, or while the specific differentiation is 

 being advanced, and this notwithstanding that the 

 physiological change must presumably have developed 

 pari passu with the morphological, and notwithstand- 

 ing that in countless cases the former is associated 

 with every conceivable variety of the latter. 



Again, why should the physiological change be 

 thus associated with every conceivable variety of 

 morphological change? Throughout the length and 

 breadth of both vegetable and animal kingdoms we 

 find this association, in the great majority of cases, 

 where new species arise. Therefore, on the supposi- 

 tion that in all such cases the physiological change 

 has been adventitiously induced by the morpho- 

 logical changes, we have to face an apparently unan- 

 swerable question — Why should the reproductive 

 mechanism of all organic beings have been thus 

 arranged, as it were, to change in immediate response 

 to the very slightest alteration in the complex har- 

 mony of " somatic " processes, which now more than 

 ever is recognized as exercising so comparatively 

 little influence on the hereditary endowments of this 

 mechanism ? Consider the difference between a worm 

 and the bird that is eating it, an oak tree and the 

 gall-insect that is piercing it : are we to suppose that 



