4i8 THE ANATOMY OF WOODY PLANTS 



to the evolution of plants in response to climatic influences, since 

 their action is mainly negative. It is true, however, that by 

 bringing about the obliteration of important groups of plants or 

 animals greater opportunity is supplied for the surviving and more 

 adaptable forms to develop in the following warmer epochs. 



It is the more gradual and not the spasmodic refrigeration 

 which has produced perhaps the greatest effect on the organization 

 of the successive plant populations of the earth. As a preliminary 

 to the discussion of the anatomical modifications which are more 

 or less definitely correlated with climatic changes in the successive 

 geological ages, the evidence furnished by plants in regard to 

 progressive climatic cooling must be considered. This evidence 

 is of two kinds. Perhaps the most important and trustworthy 

 is derived from the organization of the secondary woods in trees 

 of the various geological periods. This testimony can hardly 

 be estimated too highly in arriving at any conclusions in regard 

 to plants as reliable indicators of climatic change. Another 

 kind of evidence is afforded by the character of the plants them- 

 selves. At the present time there are large groups of plants which 

 are of more or less definite tropical occurrence and others which 

 prevail characteristically in cooler regions. The advance of 

 tropical types toward the poles or the progress of polar plants 

 in the direction of the equator in earlier geological eras must, 

 other things being equal, indicate variations in climatic conditions 

 in the direction either of greater or of less warmth, as the case 

 may be. Unfortunately, in earlier geologic times the differen- 

 tiation between polar and equatorial types was not nearly so 

 marked as it is at present, and this general situation militates 

 more or less strongly against reliable conclusions in regard to 

 climate in all but the latest geological eras. Since the present 

 work deals with anatomy, the subject of plant geography in rela- 

 tion to the climatic changes which have marked the successive 

 ages of the earth is obviously of less importance, aside from the 

 limitations indicated above. 



As has been already shown, the organization of the secondary 

 wood in extinct plants furnishes the most reliable evidence as 

 to the climatic conditions which prevailed in earlier geological 



