lyS BIOLOGICAL LECTURES. 



can be defined at all it will be possible to recognize them under 

 all conditions of growth and economic application. How far 

 such a view may be justified will become apparent upon a care- 

 ful examination of the generic and specific diagnoses.^ 



With respect to fossil plants, experience shows that the con- 

 ditions of preservation are extremely varied, so that while a 

 lignite from any given formation may have its structure per- 

 fectly preserved, another lignite from a much more recent 

 deposit may show but few of those structural features upon 

 which distinction of species may be supposed to rest. 



In accordance with these considerations, it was originally 

 held that any such classification, to be most efficient for all pur- 

 poses thus indicated, must permit conclusive deductions to be 

 drawn, if possible, from sections of about one centimeter square 

 — such as might be prepared in the ordinary way for micro- 

 scopic purposes — since this alone would meet the average 

 requirements of material derived from all sources, and more 

 particularly of material representing fossil plants. It is to be 

 observed, however, that such limitations at once impose diffi- 

 culties which, joined to those due to the fact that the wood 

 alone furnishes the necessary data, might tend to render the 

 classification of inferior value in actual practice. The aim has 

 been, therefore, to select, if possible, those distinguishing char- 

 acters which may be found in the structure of the woody parts 

 of the stem as exposed in the usual planes of section, — trans- 

 verse, radial, and tangential, — and to obtain conclusive proof 

 as to their efficiency or inefficiency for the purpose stated. 

 The results so far reached seem to justify the conclusion that 

 for genera the characters are well defined and admit of the 

 recognition of such groups without any question; while for 

 most species they present no greater difficulties than are to be 

 met with under the methods now in vogue. 



With these thoughts in mind, attention was directed in the 

 first instance toward the accumulation of authentic material — 

 a work of slow progress, now extended over a period of sixteen 

 years and, with respect to some of the angiosperms, not yet 



1 For a full account of generic characters see Trans. R. Soc. Can., Ser. 2, II, 



