Bpecies, circumscribed anew on this occasion ; especially as the appli- 

 cation of these principles involved the suppression of numerous plants, 

 to which specific value had hitherto been unquestionably assigned. 

 The description of a genuine species clearly should be so framed, as 

 to admit of its embracing any of the aberrations from the more usual 

 type, to which under various climatic or geologic circumstances a 

 species can possibly be subject ; and the diagnosis should be so con- 

 structed as to include all the cardinal characters of the species, none 

 of these ever admitting of exceptions. 



But the material even in the greatest museums and the evidence 

 extant from the most extended field-observations are as yet insuf- 

 ficient for fixing finally the diagnosis of almost any known species ; 

 and it will be therefore a problem of future research, to trace anew 

 the specific demarcation of the organic forms throughout the creation, 

 to ascertain to what extent nature has. endowed each to accommodate 

 itself to altered conditions, to elucidate how far in each special 

 instance such influences change the external form of the species, 

 and to determine the bearing of each to the geological features of 

 the globe. From his own lengthened observations, carried on in this 

 sense, the author feels justified in drawing the conclusion, that the 

 number of species has been vastly overrated, and further, that their 

 distinction never rests on a solitary or on faint characters. A study 

 of plants growing in localities, where they are exposed to most 

 unusual agencies, yields results of profound significance; and the 

 revelations, to be derived from a clear insight into the vegetation 

 of Australia, are in many cases as startling as replete with deep 

 instructive meaning. It is there where we may trace plants, often 

 in strangely altered forms from the glacier-regions to forest depres- 

 sions, in the mild air of which even tropical species may luxuriate ; 

 it is there again where we may witness the effects of the Sirocco of 

 a desert-country on otherwise alpine or tropical jungle-plants. 



But recognizing this wonderful adaptability of the species to 

 sometimes singularly different circumstances, the writer has never 

 been led to assume, that limitation of species is hopeless, or that an 

 uninterrupted chain of graduations absolutely connects the forms of 

 the living creation. Analytical dissections, counting by hundreds of 

 thousands, instituted as well on living plants in the field as on the 

 material accumulated in his museum, have never left such impres- 



