GENERAL LAWS OF ORIGIN AND MIGRATION. 259 



feet examples than those usually to be found in museums. 

 When structures are taken into the account, as well as 

 external forms, we can also depend more confidently on 

 our results. Further, the abundance of specimens to be 

 obtained in particular beds often goes far to make up for 

 their individual imperfection. The writer of these pages 

 has been enabled to avail himself very fully of these advan- 

 tages ; and on this account, if on no other, feels entitled 

 to speak with some authority on theoretical questions. 



It is an additional encouragement to pursue the sub- 

 ject, that, when we can obtain definite information as to 

 the successive floras of any region, we thereby learn much 

 as to climate and vicissitudes in regard to the extent of 

 land and water ; and that, with reference to such points, 

 the evidence of fossil plants, when properly studied, is, 

 from the close relation of plants to those stations and 

 climates, even more valuable than that of animal fossils. 



It is necessary, however, that in pursuing such in- 

 quiries 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 gen- 

 eral principles, which I think important for our guidance. 



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 er- 

 roneous 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 



