748 Evolution in the Vegetable Kingdom. ` (August; 
two favorable periods for the preservation of vegetable remains 
—the Carboniferous and the Miocene—the wide interval between 
which is relieved by two less favorable periods culminating in the 
Oolite and the Cenomanian respectively. To what extent the in- 
tervals of great scarcity may yet be filled, it is impossible to pre- 
dict, but it is well to remember that it is only quite recently that 
the Oolite has assumed prominence as a vegetable deposit, and 
this chiefly through researches made in India and Siberia. With 
the further development of such outlying regions it is to be 
hoped that a much greater degree of uniformity in the different 
geological periods will be secured. But of this there is no cer- 
tainty, and it is perhaps equally probable that future research may 
_ tend to exaggerate the present extremes. 
Three things must combine for the successful development of 
a fossil flora in any given geological formation: 1. The requi- 
site vegetation must have flourished at the period in question; 2. 
the conditions for its preservation and subsequent exposure must 
have existed ; and 3. the localities in which it is imbedded must 
be found and worked. As regards the first of these conditions, 
we know that great fluctuations of the land surface of the globe 
have taken place, and periods may have been passed during 
which these were much less in amount than at others. Still, 
there can be little doubt that the variety at least, if not the abun- - 
dance of vegetation, has undergone a. somewhat uninterrupted 
increase since the earliest times. The second condition is a much 
more serious one. Immense periods may have elapsed without 
. any record being made, not because vegetation was scarce, nor 
because land areas were limited, but because, as seems now to be 
the case over most of the globe, all vegetation was allowed to 
decay and return to the atmosphere. Again, vast beds may have 
been deposited but never afterward raised up and exposed, and 
may remain forever inaccessible. It is only the third condition 
__ which it is within the power of man to influence. But when we 
consider the accidental manner in which a great part of such dis- 
_coveries have been made thus far, we may well presume that the 
most precious scientific treasures which the earth holds may 
remain undiscovered indefinitely although within the easy reach 
of the investigator, 5: 
eo : III. Boranicar View. 
~ Most of the plants of the Paleozoic age belong to archaic 
_ types long since extinct and having only very much modified 
