230 BOTANY: PRINCIPLES AND PROBLEMS 



Geological Evidence. — Probably of most importance is the 

 existence of fossils, the actual remains or impressions left in 

 the rocks by ancient plants and animals, caught and embedded in 

 the sand or mud millions of years ago. As our knowledge of 

 geology becomes greater, we find that fossils do not occur indis- 

 criminately but that similar types appear in rock layers which we 

 know, from their position, to be of about the same age. We 



Fig. 130. — A, leaf of a fossil species of the Judas Tree (Cercis), from the Eocene 

 of Tennessee. B, leaf of a living species, Cercis canadensis, now growing through- 

 out the eastern United States. The two species are similar to each other but are 

 clearly distinct. Our living species has probably been evolved from an ancestor 

 much like the fossil one here shown. {After Berry). 



are able to assign each rock layer, or stratum, to its particular 

 level in the great series which records geological history from a 

 very remote past to the present, and we find as we pass upward 

 through this series, from the most ancient rocks to the most 

 recent ones, that the fossil remains change progressively as we 

 proceed; and that as we approach modern times, the proto- 

 types of our familiar plants and animals begin to come into view 

 (Fig. 130) until in recent deposits we find as fossils species which 

 still exist. There are enormous gaps in this record but the 

 advance of geological science is slowly filling them in, and even 

 now we can catch a glimpse of the main scenes in the pageant of 

 evolutionary progress. Among the members of the plant king- 

 dom, we can witness the rise, luxuriance, and extinction of several 

 great groups; we can trace the development of seed plants from 

 lowly, fern-like forms, and we can recognize approximately the 



