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2 



SUPPLEMENTARY SECTION. 



It is necessary, however, that in pursuing such enquiries we should have 

 some definite views as to the nature and permanence of specific forms, 

 whether with reference to a single geological period, or to successive 

 periods ; and I may be excused for stating here some general principles, 

 which I think important for our guidance, with special reference to the 

 palaeozoic floras which form the subject of this memoir, 



(1.) Botanists proceed on the assumption, vindicated by experience, 

 that, within the period of human observation, species have not materially 

 varied or passed into each other. We may make, for practical purposes, 

 the same assumption with regard to any given geological period, and may 

 hold that for each such period there are specific types, which, for the 

 time at least, are invariable. 



(2.) When we inquire what constitutes a good species for any given 

 period, we have reason to believe that many names in our lists represent 

 merely varietal forms or erroneous determinations. This is the case even 

 in the modern flora ; and in fossil floras, through the poverty of specimens, 

 their fragmentary condition and various states of preservation, it is still 

 more likely to occur. Every revision of any group of fossils detects 

 numerous synonyms, and of these many are incapable of detection without 

 the comparison of large suites of specimens. 



(3.) We may select from the flora of any geological period certain 

 forms, which I shall call specific types, which may for such period be 

 regarded as unchanging. Having settled such types, wo may compare 

 them with similar forms in other periods, and such comparisons will not be 

 vitiated by the uncertainty which arises from the comparison of so-called 

 species which may, in many cases, be mere varietal forms, as distinguished 

 from specific types. Our types may be founded on mere fragments, pro- 

 vided that these are of such a nature as to prove that they belong to dis- 

 tinct forms which cannot pass into each other, at least withia the limits of 

 one geelogical period. 



(4.) When we compare the specific types of one period with those of 

 another immediately precedent or subsequent, we shall find that some con- 

 tinue unchanged through long intervals of geological time, that others are 

 represented by allied forms regarded either as varietal or specific, and as 

 derived or otherwise, according to the view which we may entertain as to 

 the permanence of species. On the other hand, we also find new types 

 not rationally deducible on any theory of derivation from those known in 

 other periods. Farther, in comparing the types of a poor period with those 

 of one rich in species we may account for the appearance of new types in 

 the latter by the deficiency of information as to the former ; where many 





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