306 THE CELL 



see that the male and female sexual cells may be derived from the 

 same sexual apparatus, i.e. from one blossom, or they may spring 

 from different blossoms of the same shoot, or, finally, from different 

 shoots; in this way, three different degrees of relationship are 

 obtained. In hermaphrodite animals they may belong to the 

 same individuals, or to different individuals of the same species. 



The degree of relationship is still more distant when the sex- 

 ual products are derived from two different individuals of the 

 same species. In such cases also, many degrees of relationship 

 are possible, according to whether the producing individuals are 

 descendants of common parents, or are more distantly related. 

 Finally, we may have the union of sexual products derived from 

 parents which differ so much in their organisation, that they have 

 been classified as varieties of a species, or as belonging to different 

 species, or even to different genera. 



The innumerable possibilities, which the above-mentioned 

 series affords, are generally treated under three heads : (1) self- 

 fertilisation and in-breeding, (2) normal fertilisation, and (3) 

 hybridisation. There are, however, great differences of opinion 

 concerning the classification of individual cases under one or 

 other of the three heads. Further, there is no rule by means 

 of which we can estimate the various degrees of relationship of 

 the sexual cells, and which is equally applicable to all members of 

 the organic kingdom. 



A review of the facts connected with the subject teaches us, 

 that when the relationship of the reproductive cells I use the 

 expression, relationship, in its widest sense is either too near 

 or too distant, sexual affinity is either lessened or entirely done 

 away with ; therefore we may state, as a general rule, that a 

 moderate degree of relationship, which is more or less distant 

 according to the species, is the one most likely to render fertili- 

 sation possible. 



Further, we may also notice here, that sexual affinity is 

 affected by the environment. We will first discuss the ques- 

 tion of self-fertilisation, then that of hybridisation, and finally 

 we will investigate the influence exerted by the environment 

 upon these two. 



a. Self-fertilisation. Self-fertilisation occurs under the 

 most various conditions. In many cases there is no sexual 

 affinity between cells needing fertilisation, which are nearly re- 

 lated to one another, being derived more or less directly from 



