Ch. XVI.] FERTILISATION OF FLOWERS. 311 



The increased vigour resulting from cross-fertilisation is 

 allied in the closest manner to the advantage gained by change 

 of conditions. So strongly is this the case, that in some 

 instances cross-fertilisation gives no advantage to the off- 

 spring, unless the parents have lived under slightly different 

 conditions. So that the really important thing is not that two 

 individuals of different blood shall unite, but two individuals 

 which have been subjected to different conditions. We are 

 thus led to believe that sexuality is a means for infusing vigour 

 into the offspring by the coalescence of differentiated elements, 

 an advantage which could not accompany asexual repro- 

 ductions. 



It is remarkable that this book, the result of eleven years 

 of experimental work, owed its origin to a chance observation. 

 My father had raised two beds of Linaria vulgaris — one set 

 being the offspring of cross and the other of self-fertilisation. 

 The plants were grown for the sake of some observations on 

 inheritance, and not with any view to cross-breeding, and he 

 was astonished to observe that the offspring of self-fertilisation 

 were clearly less vigorous than the others. It seemed incredible 

 to him that this result could be due to a single act of self- 

 fertilisation, and it was only in the following year, when 

 precisely the same result occurred in the case of a similar 

 experiment on inheritance in carnations, that his attention was 

 " thoroughly aroused," and that he determined to make a series 

 of experiments specially directed to the question. 



The volume on Forms of Flowers was published in 1877, and 

 was dedicated by the author to Professor Asa Gray, " as a small 

 tribute of respect and affection." It consists of certain earlier 

 papers re-edited, with the addition of a quantity of new matter. 

 The subjects treated in the book are : — 

 (i.) Heterostyled Plants. 



(ii.) Polygamous, Dioecious, and Gynodicecious Plants. 



(iii.) Cleistogamic Flowers. 



The nature of heterostyled plants may be illustrated in the 

 primrose, one of the best known examples of the class. If a 

 number of primroses be gathered, it will be found that some 

 plants yield nothing but " pin-eyed " flowers, in which the 

 style (or organ for the transmission of the pollen to the ovule) 

 is long, while the others yield only " thrum-eyed " flowers 

 with short styles. Thus primroses are divided into two sets 

 or castes differing structurally from each other. My father 

 showed that they also differ sexually, and that in fact the bond 

 between the two castes more nearly resembles that between 

 separate sexes than any other known relationship. Thus for 



