voted some years to a study of these matters in such creatures that I venture to speak to you on this subject.
You remember that one of the famous early essays of Weismann was upon the question I have just raised. He tried to show that death is not at all necessarily involved in living; that natural death originally did not exist, and does not exist now in these lower creatures; with theology he held that death was acquired in the course of time, and the Satan that "brought death into the world and all our woe" was no other than natural selection, acting for the benefit of the race, as distinguished from that of the individual. The body in the course of time becomes worn, battered, crippled. It is well to have at intervals a clearing out of this worn stock; new, fresh bodies replace the battered ones and a race which undergoes regularly this renewal must prevail and perpetuate itself in the place of those that do not; such is the conception of Weismann. Thus, too, the sum of happiness in the world is kept at the highest mark, since the fresh and perfect can enjoy much more than the worn and crippled.
But according to this view, if organisms could but live in such a way as to keep the body fresh and uninjured, there would be no need for death. And the organisms which have succeeded in doing this are the infusoria and their relatives. These, in the famous phrase of Weismann, are "potentially immortal."
But another fact in the lives of these creatures attracts strongly the attention of the observer. These same unicellular organisms that appear to live forever do likewise go through the same process of sexual union that we find in higher animals. Now this sexual union has proverbially stood as the token of mortality; it is the preparation for the new generation, and prefigures the disappearance of the old one. You will recall the famous remark of Alexander the Great upon this point.
Why then should this take place in these ever-living creatures? The fact that it does was held by many to indicate that to consider these creatures ever-living was a mistake; they predicted that these animals would be found not potentially immortal, but subject to death at the end of a certain term, just as are higher animals. It is interesting to discover here, as in so many other cases, that the diverse possible opinions on the subject were formulated and maintained before investigation had obtained evidence as to the facts in the case.
But men were not content to speculate; and Maupas in one of the great investigations of biology (1883 to 1888) undertook to determine the truth of the matter. We must look briefly at the questions which were raised, and the answers that were obtained by Maupas and by others, for it will help us to understand the present state of the matter.
Maupas took a single individual (a Stylonychia), kept it with plenty of food, and allowed it to multiply by repeated division into two;
