1908.] IMPROVEMENT OF THE POTATO. 413 



BIOLOGICAL EVIDENCE 



It has long been believed by a number of investigators that a 

 conjunction of paternal and maternal nuclei is necessary for the 

 "rejuvenescence of vigor" in the species. Life has been considered 

 to be a cycle, running from conjugation to conjugation through a 

 greater or less number of generations. This was considered by 

 many to be definitely proved when Maupas (72, 73) showed that 

 colonies of Infusorians, when artifically prevented from conjugat- 

 ing, invariably died out although often several hundred generations 

 intervened. Later experiments along the same line by Calkins, 

 however, have shown that a change in diet and the stimulus of a 

 supply of chemical salts appear to be all that is necessary for con- 

 tinued propagation of Infusorians without conjugation. An addi- 

 tion of an extract of sheeps brains, was all that was necessary to 

 restore his colonies to full vigor after the 62Oth generation. 



The classical experiments of Tichomiroff and Loeb have shown 

 that artificial parthenogenesis may be induced by both mechanical 

 and chemical stimuli; while Boveri and Delage have developed 

 even non-nucleated ovum fragments to the larval stage. As one re- 

 sult of these facts, we must conclude that fertilization produces two 

 results : a. A combination of hereditary qualities ; b. A physiolog- 

 ical stimulus to growth. But since other stimuli are found to pro- 

 duce cell division, it is hardly reasonable that highly specialized 

 sexual processes should have been developed with the second result 

 as their primary objects. Indeed Weismann (105 v. i p. 343) has 

 concluded that the sole immediate effect of conjugation is "the com- 

 bination of the hereditary tendencies of two individuals into one." 



It appears that we have no data among wild plants from which 

 we are compelled to conclude that continuous bud propagation is 

 opposed to any natural law. Vines* writing of the Basidiomycetes 

 says "These fungi are not only entirely asexual but it would appear 

 that they have been evolved in a purely asexual manner from asex- 

 ual ascomycetous or aecidiomycetous ancestors. The basidiomycetes, 

 in fact, afford an example of a vast family of plants of the most 

 varied forms and habits, including hundreds of genera and species, 

 in which, so far as minute and long continued investigations have 

 shown, there is not and probably never has been, any trace of a sex- 

 ual process." 



Late cytological investigations have shown sexual processes, or 

 at least nuclear fusions similar to those of the higher plants and 



*Vifles, Nature 11: p. 625, quoted by Reid: Principles of Heredity. 



