138 THE TREES AND SHRUBS [LECT. 



tion, at the meeting held in Bath y , and it may not 

 be out of place to recapitulate here the arguments 

 which have led me to suspect, that even when the 

 external conditions remain unchanged, each species, 

 like every individual belonging to it, has its days 

 numbered, and that the period assigned to its dura- 

 tion may be extended indeed by favourable, and 

 abridged by unfavourable external conditions, but 

 in no case can transcend certain definite limits. 



I there remarked, that we seem in this instance 

 to trace the workings of two antagonistic prin- 

 ciples ; the first, that which aims at handing 

 down to the offspring the leading characteristics 

 of its parents ; the second, one which causes the 

 vigour of the race gradually to decline, and its 

 peculiar excellences to be effaced, owing to the 

 balance, upon which the harmonious workings of 

 the system depend, being destroyed, through the 

 undue preponderance of one element, and the dimi- 

 nution or loss of another. 



In both instances, however, nature seems to have 

 provided means for postponing this inevitable ter- 

 mination for a longer or shorter interval of time ; 

 namely, by those variations from the primitive type, 

 which are to a certain extent brought about by the 

 mere process of sexual reproduction, and which are 

 still further secured by those contrivances for pre- 

 venting self-fertilization, to which Mr. Darwin and 

 others have of late called particular attention. 



"> This paper is published in extenso in the " Gardener's Chronicle" 

 for October, 1864. 



