648 LECTURE XXIII. 



from distinct individuals. Amongst Phanerogams, Darwin 

 has ascertained that in many cases the pollen of one flower 

 is incapable of fertilising the oospheres of its own ovules, and 

 that the pollen from another flower of the same plant is only 

 slightly, if at all, more potent. The pollen from a flower 

 of another individual of the same species is potent, and this 

 the more so the wider the difference between the individuals : 

 the pollen from an individual of a different variety is more 

 potent than that from an individual of the same variety. 

 This physiological relation is well illustrated by Darwin's 

 observations on heterostyled plants. Not only is the structure 

 of these flowers such as almost certainly to ensure the convey- 

 ance of the pollen from anthers of a particular height to 

 the stigmas of styles of corresponding length, but the pollen 

 from other, shorter or longer, stamens is either altogether 

 impotent, or its fertilising action is much feebler, so that the 

 number of seeds produced by an illegitimate union is only 

 a fraction of that produced by a legitimate union. 



Though, as we have seen, cross-fertilisation is most 

 effectual when the individuals differ widely from each other, 

 there is a limit to the possibility of cross-fertilisation. Still 

 cases of cross-fertilisation between different species of the 

 same genus, and even between species assigned to different 

 genera, are on record, the products of such cross-fertilisation 

 being termed hybrids. But hybridisation is accompanied 

 by a diminished production of seeds, and in many cases the 

 hybrids produced have been found to be altogether sterile. 



Hybridisation is commonly reciprocal-, that is that the pollen of a 

 species A will fertilise the oospheres of a species B, and that the pollen 

 of B will likewise fertilise the oospheres of A. But in many cases this 

 has not been found to be the case. 



We see, then, that it is important that a certain relation, 

 a certain degree of sexual affinity, should exist between the 

 sexual reproductive cells. When the limit is overstepped in 

 the direction of either a too close or a too remote relation, the 

 union will either not take place at all, or the offspring will be 

 few and feeble. From this point of view we are able to explain 

 the apparent anomaly, to which attention has already been 



