The results of Calkins's experiments can evidently be interpreted in two ways:
1. It may be held that the depression was due to a too great uniformity in the food, or to the fact that the food and other conditions were not fully adapted to the animals: what the organisms needed was a change of diet. With frequent changes in diet, perhaps, there would be no degeneration at all. The final death would, on this interpretation, be due to the fact that the injury produced by uniform diet had gone too deep to be remedied by the means which Calkins tried.
2. But Calkins inclined, in view of the evidence then at his command, to another interpretation. This work came shortly after the first portions of Loeb's brilliant investigations on artificial parthenogenesis. Calkins interpreted his results in the light of those experiments. He held that the infusoria were really in senile degeneration, ready to die of old age. What he had done was essentially to induce artificial parthenogenesis; he had replaced conjugation by chemical means. The final death, he held, was due to the fact that conjugation could not be indefinitely thus replaced; old age finally asserted its power, and in the absence of conjugation produced death.
Now I think it will be apparent at this point that there are two independent questions involved in the investigations; to understand later work it is needful to distinguish them clearly.
1. Does multiplication without conjugation result in degeneration, senility and death? What is the actual cause of the degeneration that has been observed?
2. Does conjugation remedy this degeneration? An affirmative answer to this second question has been generally assumed. If animals degenerate and die without conjugation, then evidently conjugation must be what prevents and remedies this result; such has been the reasoning. But if this is true it must be possible to observe this effect of conjugation; we shall do well to follow the example of Maupas, and not rest till a plausible hypothesis has been transformed into an observed fact.
These two questions then suggest two lines for further work, and both of these lines have been followed.
Enriques and Woodruff have followed up the question: What is the cause of the degeneration that has been observed? I myself have pursued mainly the second question, as to the actual effects of conjugation. The results of all these investigations seem to me harmonious and to lead to definite conclusions.
Enriques, in 1903 to 1908, carried out cultural investigations which led him to the following results and conclusions:
1. If he did not take pains to keep his cultures free from the products of bacterial action, the animals degenerated in time, just as observed by Maupas and Calkins.