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(Berichte) to be satisfied that the chemical composition aud physical properties of 

 essential oils of ascertained botanical origin often vary considerably for the same 

 species. 



Oil determinations are usually difficult to apj)ly for purposes of diagnosis. 

 Similarly, in Eadlkofer's Monograph of the Sapindacese, I find it difficult to accept 

 such characters for the genera as, "fruit contains saponine ; fruit without 

 saponiue." 



In Eucalyptus in a given species there is variation in regard to the constituents 

 of the oil. For example, as regards ^. Wilkinsoniana, E.. T. Baker : 



At many diftereiit times of the year the oil contains small quantities of Eucalj-ptol ; at other times, 

 however, some phellandrene ; it contains, moreover, a small amount of ester. 



It is an invidious task to be the judge as to the amount of chemical variation 

 which will be admitted as evidence of the validity of a botanical species. 



E. rostrata, Schlecht., var. horealis, Baker and Smith, and E. hcvopinea, 

 R. T. Baker, var. minor, E. T. Baker {E. Wilkinsoniana, E. T. Baker) have been 

 founded "on chemical evidence alone," or mainly. I do not say that morphological 

 evidence will not be forthcoming to justify this nomenclature, but it is not available 

 yet, and it has been specially sought for. There are two ways of looking at tliis 

 matter ; one way is to endeavour to ascertain the position of a plant by morpho- 

 logical methods, and then to ascertain if the evidence supplied by physiology 

 supports the view arrived at ; or physiological evidence {e.g., based on examination 

 of essential oil) may first be examined, and the morphological facts then brought 

 under review. 



Is the oil character the one incariahle ? 



In a case such as the Stringybarks (to choose one group out of many) where 

 there is an infinite gradation of forms, the result of environment, hybridisation, and 

 perhaps other causes, the suggestion that in the oil there is a master-key to tlic 

 limitation of species seems to be based on a shifting foundation. 



The suggestion that we have at length discovered a kind of philosopher's 

 stone is indeed alluring ; that ia all the manifold changes of Eucalyptus that we 

 have at length obtained a test by which we can diagnose a species in tempting, but 

 the test will only be found to be general in application, like those applied to the 

 timbers. 



As to the variation in timber in the genus, that is the experience of any man 

 who has much to do with it. I do not wish to quote the views of the busli-worker, 

 who is often ignorant and empirical ; at the same time, many of them are very 

 shrewd. Tlie timber-inspector and the timber-merchant, who have broader views, 

 however, both share this view. 



The composition of tlie oil is not the only character other than that usually 

 employed to aid the botanist in the diagnosis of Eucalypts. Eor example, I restored 

 E. exserla, E.v.M,, to specific rank, following up a clue given to me by the timber. 



