AS AIDS IN THE DETERMINATION OF SPECIES. 



61 



11. The closeness of resemblance in the sections of two allied reputed species 

 may lead to their being recognised merely as varieties ; or, on the other hand, the 

 differences between reputed varieties may be sufficiently great to suggest their 

 distinction as species. 



12. Lastly, the general agreement among the various sections, and the absence of 

 those clear distinctions which would mark off each species if quite independent, tend 

 to the conclusion that the varied species of Eucalypts may have arisen through a 

 process of evolution. After allowing for the changes which may have taken place 

 through possible hybridisation, as hinted at in the " Eucalyptographia " (Dec. 9), 

 " extreme forms," " aberrant forms," and "transition forms," so frequently referred 

 to in that work, are not thereby accounted for. While the mere existence of 

 transition forms does not prove that the transit has been accomplished through 

 descent with modification, still permanent varieties or species, with such forms 

 flanking them on every side as would be regarded as distinct species if isolated, would 

 seem to have arisen thus. Hence the proper study of the Eucalypts, like that of the 

 Platypus and Ceratodus, may add another page to Australia's contribution to the 

 evolution theory. But perhaps it is hardly necessary to formally state this conclusion 

 in the face of the recent utterance at the NeAvcastle meeting of the British 

 Association, 1889, by the distinguished President (Prof. Flower) :—" I think I may 

 safely premise that few, if any, original workers at any branch of biology appear 

 now to entertain serious doubt about the general truth of the doctrine that all 

 existing forms of life have been derived from other forms, by a natural process of 

 descent with modification." 



Since this investigation is strictly comparative, we have endeavoured to preserve 

 as much uniformity as possible, both in the sections made and the language used. 



Besides, we felt that an important principle was being tested here, and one 

 which might receive a wider application, viz,, that in many families of plants the 

 intimate structure of the leaf-stalk might be made a means or an aid in the 

 identification of species. Therefore it was necessary to select a sufficiently varied 

 number of examples, as well as a fair proportion of the entire number of known 

 species on which to base our conclusions. It will be conceded, we think, that the 

 Eucalyptus formed a sufficiently critical genus to test, and that the number of species 

 (30), forming nearly one-fourth of the whole, was fairly extensive. According to the 

 latest "Census" by Baron von Mueller, the number of Australian species of Eucalypts 

 is 134. Whatever conclusions may be drawn, the photographed sections, and the 

 accurate outlines, particularly of the wood, ought to be a decided addition to our 

 knowledge of this characteristic and peculiar form of Australian vegetation. 



