426 SENESCENCE AND REJUVENESCENCE 



occurring in game tic reproduction, the agamic process may be 

 repeated indefinitely without race senescence. The failure of 

 agamic reproduction after a larger or smaller number of agamic 

 generations is not due to the fact that there is less rejuvenescence 

 connected with it than with the gametic reproduction, but rather 

 to the fact that under the existing conditions senescence in each 

 agamic generation is not entirely compensated by rejuvenescence 

 in each reproduction, and race senescence results. In such cases 

 of course a substitution of gametic for agamic reproduction will 

 rejuvenate the race and make possible a new series of agamic 

 generations. This course has from time to time been followed with 

 the potato, when a particular race has seemed to show signs of 

 decrease in vitality or commercial value, and often with good results. 

 There is, however, every reason to believe that a change of the 

 right kind in conditions of cultivation would accomplish the same 

 result without breeding from the seeds instead of the tubers. 

 Doubtless the gametic process affords a less difficult and more 

 rapid method of accomplishing the desired result, but it is probably 

 not the only method. 



In many organisms, under the ordinary conditions of nature, 

 senescence is evidently not completely compensated by the reju- 

 venescence occurring in agamic reproduction, and progressive 

 senescence of the race or colony occurs. This is apparently the 

 case among both plants and animals, but, as already pointed out, 

 experimental investigation has shown for many of these cases that 

 under the proper conditions progressive senescence does not occur, 

 and these results make it probable that we shall find this true for 

 many other cases. It may be, however, that in some forms senes- 

 cence progresses in spite of agamic reproduction and independently 

 of external conditions, and if so the agamic period must in any case 

 sooner or later come to an end in such forms. Perhaps some of 

 the higher animals, where agamic reproduction occurs only as 

 polyembryony or in the early stages of postembryonic life, consti- 

 tute cases of this sort. 



The point of chief importance is, however, that the difference 

 between agamic and gametic reproduction is, as regards the rela- 

 tion between senescence and rejuvenescence, one of degree rather 



