Page:George Archdall Reid 1896 The present evolution of man.djvu/45

Rh 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 potentially immortal, for in time it or its descendants die.

If we watch low unicellular organisms of almost any 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; subsequently 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 careful 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. The 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