UROLEPTUS MOBILIS ENGELM. 465 



of ' youth ' and 275 generations during the period of ' youth. ' The 

 sum of these several figures (1100) was taken as the numerical 

 expression of the vitality of an ideal series which might be used 

 as a measure of the relative vitality of any actual series, such 

 relative vitality being expressed as a percentage of 1100. There 

 is no question of absolute or exact vitality involved, but the 

 method satisfies a real need in attempts to compare the vitality 

 of an entire series with that of another. An illustration in- 

 volving the present experiments will show the usefulness of 

 this standard of measurement. The P series was one of the 

 strongest normal ex-con jugant series that I have cultivated, 

 having a relative vitality of 95 per cent of the hypothetical 

 ideal. A filial generation was taken off when the P series was 

 135 generations old, or in the period of high vitality. This was 

 the PV series which served as a normal ex-con jugant control for 

 the first group of cut conjugant experimental series, and its 

 relative vitality was 88.1 per cent. Another filial series was 

 taken from the P series when the latter was 291 generations old, 

 or well advanced in old age; this, the a series, had a relative 

 vitality of 44 per cent and was noticeably weak throughout the 

 life-cycle. 



The first group of cut conjugant experimental series was 

 derived from individuals taken from the same parent stock and 

 at the same time as the PV series. The second and third groups 

 were taken from parent stocks when the latter were in late 

 stages of their life-cycles (203 and 270 generations, respectively). 

 In relation to relative vitality, the same effects were observed in 

 the experimental series as in the normal ex-conjugant controls; 

 those derived from young parents were more vigorous than 

 those derived from old (table 1), but, with the exception of the 

 X75 series, not one of the experimental series was as vigorous 

 as its normal ex-conjugant control — a phenomenon which may 

 have been due to the operation and subsequent reorganization, 

 to absence of amphimixis, or it ma}^ have been purely accidental. 



In regard to rejuvenescence and relative vitality, therefore, 

 the series derived from cut conjugants behaved like normal, 

 ex-conjugant series. They were likewise normal in other re- 



