PREFACE. 



xi 



at the time of the views of M. Du Petit-Thouars ; and 

 that it was subsequently more largely developed in a 

 paper On the Nature^ Longevity ^ and Size of Trees^ 

 which he published in the Edinburgh New Philoso- 

 phical Journal" for January 1847, long before he 

 had any knowledge of the writings of De la Hire, 

 Darwin, Mirbel, or Gaudichaud. 



What led the author to direct his attention to this 

 subject was, a difficulty he felt soon after he began 

 his labours as a teacher of Physiology in 1840, in 

 bringing trees within the pale of two laws universally 

 regarded as applicable to all living beings — the law 

 of a determinate duration of life, and the law of a 

 determinate size of organism. His earlier inquiries 

 disposed him to regard the difficulty as insuperable, 

 and the laws as so far exceptional. After a time, 

 however, two things drew him into the train of 

 thought out of which the theory sprang. One was, 

 reflection on the potato-plant as the produce of a 

 stem furnished with hiids^ and capable of being re- 

 produced from year to year indefinitely from buds 

 alone, and the comparison of this with the results of 



