REPRODUCTION AND DISPERSAL 821 



general, no demonstrable loss comes from its elimination in those cases 

 where vegetative reproduction is well-developed, and such cases con- 

 stitute the vast majority. In various seed plants (as the duckweeds) 

 sexual reproduction rarely occurs, and many economic plants (such as 

 the banana, the fig, and the sweet potato) have been propagated almost 

 from time immemorial solely by vegetative means and yet without obvious 

 deterioration. A second theory claims that sexuality insures rejuvenes- 

 cence. However, this is insured much more generally and economically 

 through propagation and asexual reproduction. A third theory is that 

 sexuality favors variation and therefore evolution through the repeated 

 mingling of new elements, thus giving rise to new combinations of char- 

 acters and hence to new species. This view seems reasonable, but there 

 is little positive evidence in its favor. Furthermore, variation is known 

 to be of frequent occurrence in the bacteria and blue-green algae and in 

 other sexless groups; indeed, many investigators hold that crossing 

 promotes fixity rather than variation, and it has been shown that in 

 inbred races of Spirogyra and Phaseolus the amount of variation is as 

 great as or greater than in cross-bred races. Sometimes the theory is 

 advanced that the significance of sexuality lies in the fusion of kinetic 

 and trophic (i.e. nutritive) elements; the egg is regarded as having the 

 food necessary for development, while the sperm adds the requisite 

 developmental stimulus. These kinetic and trophic roles are not to be 

 doubted, but they furnish no clew to the significance of sexuality, giving 

 rather an explanation of embryo development. In propagules and in 

 asexual spores both kinetic and trophic elements are present in sufficient 

 degree to insure development, so that in these respects sexuality adds 

 nothing new. At present no theory as to the role of sexuality has much 

 support. It is not impossible that it is a necessary accompaniment of 

 evolution but without particular significance, although in the entire 

 plant kingdom there probably is no other equally widespread phenomenon 

 which is without conspicuous advantage. The most that can be said 

 with certainty concerning the advantage of sexual reproduction among 

 the lower plants is that it supplements the other and more successful 

 kinds. 1 As to the plants above the thallophytes, there remains to be 

 considered the alternation of generations. 



Ulva, where fusion may take place between sister gametes arising from a common cell. 

 In some ferns (as Lastrea) and in various fungi, the fusing structures are thought to be 

 vegetative rather than sexual ; in contrast to amphimixis and automixis such fusion 

 has been termed pseudomixis. 



1 In Paramoecium, one of the infusorians, individual animals reproduce ordinarily by 



