48 THE LAW OF BIRTHS AND DEATHS 



remarkable uniformity. It may be noted here that the 

 theory is a theory of fertilisation, and, for reasons to be 

 made clear later, has no application to asexual repro- 

 duction. 



Darwin, in his Variation of Animals and Plants under 

 Domestication, brought together a large body of evidence 

 bearing on this question. As this body of evidence repre- 

 sents the observations of many years, and as Darwin was 

 the most indefatigable and reliable of observers, we may 

 fairly take it as accurately representative of the facts 

 as a whole. The following extracts will show that the 

 general rules deduced from the principle are fully borne 

 out by the facts. 



"It is notorious that many plants in our gardens and 

 hot-houses, though preserved in the most perfect health, 

 rarely or never produce seed. I do not allude to plants 

 which run to leaves, from being kept too damp, or too 

 warm, or too much manured ; for these do not flower, 

 and the case may be wholly different. Nor do I allude 

 to fruit not ripening for want of heat, or rotting from 

 too much moisture. But many exotic plants, with their 

 ovules and pollen appearing perfectly sound, will not 

 set any seed. The sterility in many cases, as I know 

 from my own observation, is simply due to the absence 

 of the proper insects for carrying the pollen to the stigma. 

 But after excluding the several cases just specified, there 

 are many plants in which the reproductive system has 

 been seriously affected by the altered conditions of life 

 to which they have been subjected." 1 



Darwin then proceeds to give many instances showing 

 the sterility of wild plants when transplanted to cultivated 

 gardens, together with exceptions to this rule. He gives 

 no instance of a plant whose fertility is directly increased 



1 Variation of Animals and Plants under Domestication, chap, xviii. 



