Self-fertilisation 421 



While the influence of the work on the biology of flowers was 

 extraordinarily great, it could not fail to elicit opinions at variance 

 with Darwin's conclusions. The opposition was based partly on 

 reasons valueless as counter arguments, partly on problems which 

 have still to be solved ; to some extent also on that tendency against 

 teleological conceptions which has recently become current. This 

 opposing trend of thought is due to the fact that many biologists 

 are content with teleological explanations, unsupported by proof ; 

 it is also closely connected with the fact that many authors estimate 

 the importance of natural selection less highly than Darwin did. 

 We may describe the objections which are based on the wide- 

 spread occurrence of self-fertilisation and geitonogamy as of little 

 importance. Darwin did not deny the occurrence of self-fertilisation, 

 even for a long series of generations ; his law states only that 

 "Nature abhors perpetual self-fertilisation 1 ." An exception to this 

 rule would therefore occur only in the case of plants in which the 

 possibility of cross-pollination is excluded. Some of the plants with 

 cleistogamous flowers might afford examples of such cases. We have 

 already seen, however, that such a case has not as yet been shown to 

 occur. Burck believed that he had found an instance in certain 

 tropical plants (Anonaceae, Myrmecodia) of the complete exclusion 

 of cross-fertilisation. The flowers of these plants, in which, however, 

 in contrast to the cleistogamous flowers the corolla is well 

 developed, remain closed and fruit is produced. 



Loew 2 has shown that cases occur in which cross-fertilisation 

 may be effected even in these " cleistopetalous " flowers : humming 

 birds visit the permanently closed flowers of certain species of 

 Nidularium and transport the pollen. The fact that the formation 

 of hybrids may occur as the result of this shows that pollination may 

 be accomplished. 



The existence of plants for which self-pollination is of greater 

 importance than it is for others is by no means contradictory to 

 Darwin's view. Self-fertilisation is, for example, of greater im- 

 portance for annuals than for perennials as without it seeds might 

 fail to be produced. Even in the case of annual plants with small 

 inconspicuous flowers in which self-fertilisation usually occurs, such 

 as Senecio vulgaris, Capsella bursa-pastoris and Stellaria media, 

 A. Bateson 3 found that cross-fertilisation gave a beneficial result, 



1 It is impossible (as has been attempted) to express Darwin's point of view in a single 

 sentence, such as H. Miiller's statement of the " Knight-Darwin law." The conditions of 

 life in organisms are so various and complex that laws, such as are formulated in physics 

 and chemistry, can hardly be conceived. 



2 E. Loew, "Bemerkungen zu Burck...," Biolog. Centralbl. xxvi. (1906). 



3 Anna Bateson, " The effects of cross-fertilisation on inconspicuous flowers," Annals of 

 Botany, Vol. i. 1888, p. 255. 



