THE VITAL PROPERTIES OF THE CELL 309 



and the presence of strong mutual attraction in nearly related 

 sexual cells there are many gradations. 



Amongst the numerous egg-cells which are contained in an 

 ovary, only a few develop and become ripe seeds, where self- 

 fertilisation with the pollen of the same flower is induced 

 artificially. From this we may conclude that the individual 

 egg-cells possess somewhat different sexual affinities ; that is 

 to say, that whilst some may be fertilised with the pollen 

 of their own flower, others cannot ; thus they exhibit differences 

 similar to those which we shall come across in hybridisation. 



Finally, it may be possible for egg-cells to be fertilised, to 

 begin to develop, and then to die off prematurely. In support 

 of this view, the phenomenon may be quoted, that many flowers, 

 which have been induced artificially to fertilise themselves, fade 

 more quickly than those which have been fertilised in a natural 

 manner. Indeed, the flowers of certain Orchids become black 

 .and necrotic when treated in this fashion. This is probably 

 due to the premature death and disintegration of the embryos 

 which were about to be developed (Darwin VII. 8). 



The seeds, which develop as a result of self-fertilisation, fre- 

 quently produce only weakly plants, showing some defect or 

 other in their constitution ; further, the pollen grains are often 

 imperfectly developed. 



From these three facts, namely, that in many organisms nearly 

 related sexual cells do not combine ; that in others, even if fertili- 

 sation does take place, the embryo is arrested in its development, 

 and soon dies ; and that finally, even if development proceeds 

 uninterruptedly, the evolved organisms are weakly ; we are able 

 to draw the general conclusion, that self-fertilisation on the 

 whole acts disadvantageously. It is true, that in individual 

 cases this disadvantage cannot be perceived, yet these excep- 

 tions do not disprove the accuracy of this statement any more 

 than the occurrence of parthenogenesis can be taken as an 

 argument against the theory, that great advantage is to be 

 derived from fertilisation. 



That there must be something detrimental in self-fertilisation i 

 may be inferred from a cursory glance over the organic kingdom. 

 As Darwin (VII. 8) says, nature evidently abhors frequent self- / 

 fertilisation, for we see constantly on every side, that most com- 

 plicated arrangements have been made in order to prevent itsj 

 occurrence. 



