INTRODUCTION. v 



There can hardly be a question with regard to Professor 

 Gray's value as a critic. His reviews represented the opinion 

 of a just and discriminating mind, thoroughly familiar with 

 all sides of the question before it, critical rather than lauda- 

 tory, loving the truth and its investigators, but the truth above 

 everything else. Xo other naturalist of his reputation and 

 attainments ever devoted so much time to literary work of this 

 sort, or continued it so uninterruptedly for so many years ; 

 and in our time the criticism and advice of no other botanist 

 has been so eagerly sought or so highly valued by his contem- 

 poraries. 



The selection of the articles for republication has been an 

 embarrassing and difficult task. The amount of material at 

 my disposal has been overwhelming, and desirable as it might 

 have been to republish it all, it has not been possible to do 

 so within reasonable bounds. More than eleven hundred bib- 

 liographical notices and longer reviews were published by 

 Professor Gray in different periodicals ; and it was necessary 

 in preparing these volumes to exclude a number of papers 

 of nearly as great interest and value as those which are 

 chosen. 



I have endeavored in making this selection to present, as far 

 as it is possible to do so in a series of papers written indepen- 

 dently of each other during a period of more than fifty years, 

 a history of the growth of botanical science during a period 

 which must remain one of its great eras — a period marked 

 by the gradual change of ideas among naturalists upon the 

 origin and fixity of the species which has broadened the field 

 of all biological investigation ; by the establishment and sys- 

 tematic arrangement of vast herbaria gathered from all parts 

 of the world ; by the introduction of improved and more phil- 

 osophical methods of investigation in the laboratory ; and by 

 the growth of popular appreciation for the value of scientific 

 training. I have tried, in making a selection of these articles, 

 to display as far as possible the mental grasp of their author 

 and his varied attainments in all departments of botany ; and 

 to include the reviews of those works which Professor Gray 

 himself believed had played in the two continents, during his 



