414 BULLETIN No. 127. [August, 



animals, in some of these fungi; but there are other species, and 

 among animals, some species of crustaceans, which reproduce ex- 

 clusively by parthenogenesis. Some of the fungi are thought to 

 have vestiges of degenerate sexual organs, and in the Crustaceans 

 it can be demonstrated (105) that they once reproduced sexually, 

 by their possession of the sac which once served for receiving 

 the spermatozoa. It is extremely improbable that a process neces- 

 sary or even advantageous to the continuance of any species could 

 have been allowed to degenerate under the operation of natural se- 

 lection. 



In higher plants we have numerous examples where either no 

 seed is produced or where seed propagation is seldom resorted to 

 and yet we hear no serious charges of degeneration. Among them 

 may be given the banana, hops, strawberry, sugar cane and many 

 of the grasses. There are also 1 certain parthenogenetic plants, such 

 as the dandelion, that are certainly in no danger of dying out from 

 their method of seed production. 



The fact of degeneration in potatoes seems to have been ex- 

 plained as variety senility due to bud propagation, as a convenient 

 prop to various hypotheses as to the function of sex; and this con- 

 clusion on theoretical grounds is decidedly unproved. 



EXPERIMENTAL EVIDENCE 



So far as I am able to learn the senility theory was first pro- 

 pounded about 1806 by Knight (61) although he did not lay special 

 stress on potatoes. His hypothesis was that all varieties propagated 

 by buds (particularly fruits) have a most productive period when 

 they are of middle age and then become "subject at no very distant 

 period to the debilities and diseases of old age." 



Aitken (i) first applied the hypothesis to potatoes in 1837. 

 He believed that although most fruits were produced from seed 

 each year, a potato variety was a single plant propagated yearly 

 through its tubers, and must grow old in the process. The first 

 sign of this weakening of the variety, he says, is a lack of blossoms. 



In Germany, Berchtold (9) in 1842 accepted Aitken's view with 

 limitations. He considered that disease, climate, soil, cultivation 

 and other conditions were the important factors to be considered. 



Later Heine (53) of Emersleben, who was for ten years the 

 chief German writer on potatoes, was very pronounced in his ac- 

 ceptance of Aitken's theory, and was followed by many of his 

 countrymen. He speaks continually of "the unalterable law of na- 

 ture that only through sexual seed propagation is it possible to 



