522 Fl.ORA OF THE I.ARAMIK GROUP. 



many of these are so similar to liviug genera as to be designated by 

 the same names with moditied terminations, such as iten, opsis, etc., and 

 such forms are, with better material and more careful study, being cou- 

 stantly made to take their places as true living genera. The vertebrate 

 I)ak'outologist, therefore, deals with genera as the paleobotanist does 

 with species, and in fact, as is well known, in this department of zoology 

 the term "genus" is given a much more limited meaning than it is in 

 botany, and a rank not far above that of " species" among plants. This 

 is doubtless in great i)art necessary, and due to nature having drawn 

 classificatory lines, so to speak, at somewhat different i)oints in different 

 scales of being. But it is clear that the paleobotanist cannot compare 

 his genera as the vertebrate paleontologist compares his for the settle- 

 ment of questions of geologic age. It is, however, true that certain 

 genera which flourish at the present day pretloininate in certain forma- 

 tions and are rare or absent in others of later age, so as in a true sense 

 to be characteristic of such formations. This does not prove that they 

 subsequently dwindled away and then revived at a still later date, 

 although this might, and jirobably sometimes does, occur. But the ex- 

 planation is that several beds of difterent age are usually in different 

 parts of the world, and the flora of the globe in past time, as at present, 

 has sustained different types of vegetation at difterent points on its 

 surface. Or, if the beds are neaily over each other, /. e., not far sep- 

 arated geographically, the predominance of certain genera in lower that 

 are rare or absent in higher strata must be explained on the hypothesis 

 of migration or by supposing that the nature of the country at the two 

 points was very different at the time of the respective deposits. It 

 thus comes about that when we speak of the Laramie flora we refer to a 

 definite geographical area at a definite period of time, and when we speak 

 of the Eocene flora we mean the beds occurring at the localities named 

 on our table and a few others grouped together in the last column but 

 one. If the reader will take the trouble to inspect the columns of the 

 table in which the Senonian species are set down he will find that a very 

 marked distinction exists between those of Europe on the one hand and 

 those of America and the Arctic regions on the other, and that the lat- 

 ter resemble much more closely those of the Laramie group. This is 

 entirely because they are in nearer geographical relationship with them. 

 But it must not be forgotten that genera are capable of great modi- 

 fications without rendering a change of name necessary, and the prac- 

 tice among paleobotanists has been to crowd everything into living gen- 

 era that they will contain without doing violence to their accepted at- 

 tributes. Therefore,anBoceneoraCretaceousgenus, though still living, 

 may embrace forms widely divergent from those now recognized under 

 the same name, so that such genera may really be characteristic of those 

 formations as strictly as though they had become estinctat their close. 

 The principal interest, therefore, centers upon these characteristic 



