ANIMAL AND PLANT REPRODUCTION 31 



that only an extremely small percentage of the species of 

 flowering plants which have held their own to the present 

 day in the struggle for existence, have adopted a method 

 of fertilization which permits no crossing. Some of our 

 most vigorous cultivated plants — tobacco, wheat, peas and 

 beans — are naturally and usually self -fertilized, but they 

 each and every one have their flowers so arranged as to 

 pennit an occasional cross. 



At the same time, one would be too hasty if he con- 

 cluded from these facts that continuous self-fertilization 

 or other means of reproduction which result in a single 

 line of descent is incompatible with inherent racial vigor. 

 At least, there is evidence that various species which seem 

 well able to hold their own seldom resort to crossing as a 

 means of propagation, yet one could hardly use them as 

 examples of degeneration. As illustrations, there is no 

 need to go below the flowering plants, either, although if 

 one desires an example of a long-continued evolution of 

 species and genera without any form of sexual reproduc- 

 tion he is forced to look to the Basidiomycetes. In this 

 large group the fungi are not only asexual themselves, but 

 appear to have been developed in a purely asexual manner 

 from asexual ancestors. But in the flowering plants, 

 many of our most useful types — the potato, the banana, 

 hops and sugar cane — seldom have recourse to sexual re- 

 production. It is true many agriculturists insist that 

 these species sooner or later degenerate for this very 

 reason, but they have never been able to bring forward 

 one atom of critical evidence to uphold their view. Vari- 

 eties of potatoes or of sugar cane do indeed degenerate, 

 but it is probably because of disease which from their 

 method of propagation is difficult to eradicate, and not 



