416 



Discussion. 



Subject — 'The application of "Jordan's Law" to the case of the 

 Australian fauna and flora. 



The generalisation, to which Dr. J. A. Allen has applied the 

 name of Jordan's Law, has been stated thus — "Given any species, 

 in any region, the nearest related species is not to be found in 

 the same region nor in a remote region, but in a neighbouring 

 district separated from the first by a barrier of some sort or at 

 least by a belt of country, the breadth of which gives the eflfect 

 of a barrier." 



President D. S. Jordan has given an exposition of his views, 

 in a paper entitled "The Law of Geminate Species " (American 

 Naturalist, Vol. xlii., p. 73, February, 1908). Geminate species 

 are defined as " twin species — each one representing the other on 

 opposite sides of some form of barrier. In a general way, these 

 geminate species agree with each other in all the respects which 

 usually distinguish species within the same genus. They differ 

 in minor regar^Is, characters which we mny safely suppose to be 

 of later origin than the ordinary specific characters in their 



group"(p.75) "One of the most remarkal)le cases of 



geminate species is that of the fishes on the two sides of the 

 isthmus of Panama. Living under essentially the same condi- 

 tions, but separated since the end of the Miocene Period by the 

 rise of the isthmus, we find species after species which has thus 



been split into two"(pp 75-76) "Among plants we 



often notice the fact — rare though not unknown among animals 

 — of numerous species of the same genus occupying the same 

 area. In some cases these species are closely related, suggesting 

 mutants, and in other cases the relation indicates the existence 

 of hybrids Eucalyptus, Acacia, and Epacris in Aus- 

 tralia are examples even more striking. But I have never seen 

 very closely related or geminate forms in any of these genera 

 actually growing together. 1 suspect that they do so sometimes 

 and that the explanation is found in reinvasion"(pp.78-79). 



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