128 BOOKS AND CUERENT LITERATURE 



admit at the outset that some identifications based on fragmentary 

 materials are altogether too uncertain. There is more or less conver- 

 gence in foliar characters in unrelated or remotely related families 

 and there may also be considerable variation in the leaves of a single 

 species, but foliar characters in general are more fixed than those of 

 almost any other organs of plants. They are subjected to less com- 

 plex environmental factors and always have been. It should be re- 

 membered that characters which are less essential to the vital activi- 

 ties of plants, such as the form of the leaf, when once acquired may 

 continue practically unchanged for thousands of years and afford a 

 surer clue to relationships than characters more immediately within 

 the field of action of natural selection." 



Approximately 100, or somewhat less than one-third of the species 

 listed by Berry, are forms which had previously been described by 

 Heer, Hilgard, Unger, Hollick, Knowlton, Newberry, Veatch and Les- 

 quereaux. The investigations of Berry have led him to refer approx- 

 imately 50% of these forms to different genera from those to which 

 they were assigned by previous investigators. Even more significant 

 is the fact that 40% of the forms have been transferred to different 

 orders. 



Such striking discrepancies in the identification of leaf impressions 

 by paleobotanists might easily be considered to indicate that foliar 

 characters are not to be relied upon in determining the relationships 

 of fossils. However, the reviewer is inclined to believe that these 

 discrepancies are largely due to the fact that many of the earlier deter- 

 minations were based upon more or less fragmentary material and a 

 less comprehensive study of the foliar characters of living plants. 



A considerable portion of Professor Berry's monograph is devoted 

 to the description of various orders, families and genera of the Angio- 

 sperms, particularly their present distribution and occurrence in vari- 

 ous geological formations. This portion of the volume, which has 

 been printed separately,* should prove very useful for general reference. 



It is to be hoped that Professor Berry will publish the results of his 

 detailed study of the form and venation of leaves, since an accurate 

 key to the plants of the tropics based upon vegetative characters would 

 be of much economic and scientific value. — I. W. Bailey. 



^ Berry, E. W.; The Affinities and Distribution of the Lower Eocene Floras 

 of Southeastern North America. Am. Phil. Soc, 53: No. 214, pp. 129-250, June- 

 July, 1914. 



