460 GENERAL RESULTS. Chap XIL 



a very early period of life, whilst the organisation is 

 highly flexible. We have, moreover, reason to believe 

 that changed conditions generally act differently on 

 the several parts or organs of the same individual;* 

 and if we may further believe that these now slightly 

 differentiated parts react on one another, the harmony 

 between the beneficial effects on the individual due to 

 changed conditions, and those due to the interaction of 

 differentiated sexual elements, becomes still closer. 



That wonderfully accurate observer, Sprengel, who 

 first showed how important a part insects play in the 

 fertilisation of flowers, called his book ' The Secret 

 of Nature Displayed ; ' yet he only occasionally saw 

 that the object for which so many curious and beautiful 

 adaptations have been acquired, was the cross-fertilisa- 

 tion of distinct plants ; and he knew nothing of the 

 benefits which the offspring thus receive in growth, 

 vigour, and fertility. But the veil of secrecy is as 

 yet far from lifted ; nor will it be, until we can say 

 why it is beneficial that the sexual elements should 

 be differentiated to a certain extent, and why, if the 

 differentiation be carried still further, injury follows. 

 It is an extraordinary fact that with many species, 

 even when growing under their natural conditions, 

 flowers fertilised with their own pollen are either 

 absolutely or in some degree sterile ; if fertilised with 

 pollen from another flower on the same plant, they are 

 sometimes, though rarely, a little more fertile; if 

 fertilised with pollen from another individual or variety 

 of tha same species, they are fully fertile ; but if 

 with pollen from a distinct species, they are sterile 

 in all possible degrees, until utter sterility is reached. 



* See, for instance, Brackenridge, ' Theory of Diathesis, Edinburgh, 

 186y. 



