DISCUSSION. 423 



The analysis of the 43 pairs of Odonata chosen, yields a result 

 remarkably close to this expectation, viz : — 



Inlying Pairs(10 out of 43) 23% 



Overlapping Pairs(24 out of 43) 56% 



Isolated Pairs(9 out of 43) 21% 



It is not proposed to claim the dignity of a natural law for 

 these suggestions, as it is obvious how very far from the truth 

 they might be in the case of a complicated group, such as the 

 Australian Eucalypts; where, even if the numerous species 

 could be defined satisfactorily, and the main causes that gave 

 rise to them were known, many other forces besides those here 

 taken into account, must have operated in forming them. The 

 evidence of the Australian dragonflies, therefore, is on the whole 

 distinctly adverse to the acceptance of " Jordan's Law," as 

 holding generally between allied species. In Australia, at any 

 rate, the barriers have been neither strong enough nor lasting 

 enough to have affected the majority of native forms. 



Acting-Professor S. J. Johnston — In formulating his law, 

 Jordan seems to have supposed that one of the essential factors 

 in the evolution of his geminate species from their ancestral 

 types, is Geographical Isolation. While this may be mainly true 

 in the case of the higher animals, in the case of lower animals 

 and of plants phi/siological selection is a much more potent 

 factor and may quite exclude geographical isolation; so that we 

 find two closely related species, so closely related that they may 

 be looked upon as having diverged from a common ancestral 

 type, living side by side and not separated by a geographical 

 barrier, as Jordan stal»es. As we descend in the scale of animal 

 life Jordan's Law seems to be less and less applicable, while to 

 plants it does not seem to apply at all. In plants, for instance, 

 physiological selection, working through such factors as jore- 

 pof.ency, differences in the time of ripening of flowers, small 

 differences in structure adapted to special insects, seem to be much 

 more potent in producing that divergence in structure which 

 leads to the formation of new species th^n geographical isolati 



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L I B R A R Y)3D 



