ORGANIC EVOLUTION — PHYSICAL 33 



potentially immortal. It may be killed by external 

 circumstances, by starvation, by heat, by cold, by 

 violence, &c., but if the conditions are favourable no 

 death occurs, for, as Weismann pertinently remarks, " If 

 it is mortal, if it dies — What is it that dies ? where is 

 the dead body ? " The leucocyte, however, is not poten- 

 tially immortal, for in time it or its descendants die. 



If we watch low unicellular orsranisms of almost anv 

 species we see that occasionally two of them come 

 together and fuse more or less completely, so that 

 the two animals become one, or so that an exchange 

 of substance takes place between their nuclei ; sub- 

 sequently the dual animal divides and re-divides many 

 times before fusion again occurs. But however many 

 the number of cell-divisions subsequent to fusion, recent 

 investigations seem to show that the descendants of the 

 conjugated pair ultimately perish, unless fusion again 

 occurs. 



"The riddle was in part solved by a long series of 

 carefiil observations. In November 1885, Mons. Maupas 

 isolated an infusorian ' Stylonichia Pustulata,' and 

 observed its generations till March 1886. By that time 

 there had been two hundred and fifteen generations 

 produced by ordinary division, and since these lowly 

 organisms do not conjugate with near relatives, there 

 had of course been no sexual union. 



" What was the result ? At the date referred to the 

 family was observed to have exhausted itself. Th-3 

 members were being born old and debilitated. The 

 asexual division came to a standstill and the powers of 

 nutrition were lost. 



"Meanwhile before the generations had exhausted 

 themselves several of the individuals had been restored 

 to their natural condition, where they conjugated with 

 unrelated forms of the species. One of these was 

 isolated and watched for five months and the usual 

 number of successive generations occurred. On to the 



