Evolution and Adaptation

by Thomas Hunt Morgan

Brooks made the interesting and ingenious suggestion that the separation of the sexes has been brought about as a sort of specialization of the individuals in two directions. The male cells are supposed to accumulate the newly acquired characters, and represent, therefore, the progressive element in evolution. The female cells are the conservative element, holding on to what has been gained in the past. It does not seem probable, in the light of more recent work, that this is the function of the two sexes, and it is unlikely that we could account for the origin of the two sexes through the supposed advantage that such a specialization might bring about. A number of writers, Galton, Van Beneden, Bütschli, Maupas, and others, have looked at the process of sexual reproduction as a sort of renewal of youth, or rejuvenescence of the individuals. There is certainly a good deal in the process to suggest that something of this sort takes place, although we must be on our guard against assuming that the rejuvenescence is anything more than the fulfilment of a necessary stage in the life history. Weismann has ridiculed this suggestion on the ground that it is inconceivable that two organisms, decrepit with old age, could renew their youth by uniting. Two spent rockets, he says, cannot be imagined to form a new one by combining. There is apparent soundness in this argument, if the implication is taken in a narrow physical sense. If, on the other hand, the egg is so constituted that at a certain stage in its development an outside change is required to introduce a new phase, then the conception of rejuvenescence does not appear in quite so absurd a light.

This hypothesis of rejuvenescence is based mainly on certain processes that take place in the life history of some of the unicellular animals. Let us now see what this evidence is. The results of certain experiments carried out by Maupas on some of the ciliate protozoans have been fruitful in arousing discussion as to the ultimate meaning of the sexual process. Maupas’ experiments consisted in isolating single individuals, and in following the history of the descendants that were produced non-sexually by division. He found that the descendants of an individual kept on dividing, but showed no tendency to unite with each other. After a large number of generations had been passed through (in Stylonychia pustulata, between 128 and 175; in Leucophys patula, 300 to 450; and in Onychodromus grandis, 140 to 230 generations), the division began to slow down, and finally came to a standstill. Maupas found that if he took one of these run-down individuals, and placed it with another in the same condition from another culture, that had had a different parentage, the two would unite and the so-called process of conjugation take place. This process consists for the species used, in the temporary union and partial fusion of the protoplasm of the two individuals, of an interchange of micronuclei, and of a fusion, in each individual, of the micronucleus received from the other individual with one of its own. The individuals then separate, and a new nucleus (or nuclei) is formed out of the fused pair.

The individuals in question, in which this interchange of micronuclei has taken place, undergo a change, and behave differently from what they did before. They feed, become larger and less vacuolated, and are more active. They soon begin once more to divide. Maupas found that an individual that has conjugated will run through a new cycle of divisions, which will, however, after a time also slow down, unless conjugation with another individual having a different history takes place. If conjugation is prevented, the individual will die after a time. These results seemed to show that the division phase of the life history cannot go on indefinitely, and that through conjugation the individual is again brought back to the starting-point.

Quite recently Calkins has carried out a somewhat similar series of experiments, which have an important bearing on the interpretation of Maupas’ results. The experiment of isolating an individual and tracing the career of its descendants was repeated with the following results: two series were started, the original forms coming from different localities. Of their eight descendants four of each were isolated. The remaining four of each set were kept together as stock material. The rate of division was taken as the measure of vitality. The animals divided more or less regularly from February to July. After each division (or sometimes after two divisions) the individuals were separated. About the 30th of July the paramœcia began to die “at an alarming rate, indicating that a period of depression had apparently set in, or degeneration in Maupas’ sense.” Up to this time the animals had been living in hay infusion, renewed every few days, from which they obtained the bacteria on which they feed. Calkins tried the effect of putting the weakened paramœcia into a new environment. Infusion of vegetables gave no good results, but meat infusions proved successful. “The first experiment with the latter was with teased liver, which was added to the usual hay infusion. The result was very gratifying, for the organisms began immediately to grow and to divide, the rate of division rising from five to nine divisions in successive ten-day periods.” This beneficial effect was not lasting, however, and after ten days the paramœcia began to die off faster than before, and the renewed application of the liver extract failed to revive them. A number of other extracts were then tried without effect. Finally they were transferred to the clear extract of lean beef in tap water. The effect of this medium was interesting, for, although it restored the weakened vitality, there was no rapid increase in the rate of division, as when first treated with the teased liver. The infusoria were, however, now large and vigorous, and did not die unless transferred from the beef medium to the usual hay infusion. “When this was attempted, they would become abnormally active and would finally die. The division rate gradually increased during the month of August until, in the last ten days, they averaged six generations. Finally, in September, the attempts to get them back on the old diet of hay infusion were successful, and then the division rate went up at once to twelve times in ten days, and a month later they were dividing at the rate of fifty times a month.”

“These cultures went on well until December, when the paramœcia began to die again. They were saved once more with the beef extract, and when returned later to the hay infusion continued through another cycle of almost three months. Some of these were treated, once a week for twenty-four hours, with the beef extract, and while the two sets ran a parallel course at first, those kept continuously in the hay infusion died after a time, but those that had been put once a week into the beef extract (which had been stopped, however, in March) continued their high rate of division throughout the period of decline of their sister cells, and did not show signs of diminished vitality until the first period in June.” At this time their rate of division increased rapidly. They were put back into the beef extract, but it failed now to have a beneficial effect, and the animals continued to die at a rapid rate. To judge from the appearance of the organisms the new decline was due to a different cause; for, while in the former periods the food vacuoles contained undigested food, at this period the interior was free from food masses. The protoplasm became granular and different from that of a healthy individual. None of the former remedies were now of any avail. “When the last of the B-series stock had died in the five hundred and seventieth generation (June 16th), it looked as though the cultures were about to come to an end.” Extract of the brain and of the pancreas were then tried. To this a favorable response took place at once. The organisms became normal in appearance and began to divide. After forty-eight hours’ treatment they were returned to the usual hay infusion. Here they continued to multiply and reached on June 28th the six hundred and sixty-fifth generation.

There can be no doubt that the periods of depression that appear in these infusoria kept in cultures can be successfully passed if the animals are introduced into a new environment. Without a change of this sort they will die. Calkins thinks that the effect is produced, not by the new kind of food that is supplied, but by the presence of certain chemical compounds. The beef extract “does not have a direct stimulating effect upon the digestive process and upon division, for, while the organisms are immersed in it, there is a very slow division rate; when transferred again to the hay infusion, however, they divide more rapidly than before.”

This brings us back to the idea of the “renewal of youth” through conjugation. Maupas claimed that union of individuals having the same immediate descent is profitless. Calkins suggests that this is due to the similarity in the chemical composition of the protoplasm of the two individuals. When in nature two individuals that have lived under somewhat different conditions conjugate, the result should be beneficial, since there takes place the commingling of different protoplasms.

Calkins’s work has shown that by means of certain substances much the same effect can be produced as that which is supposed to follow from the conjugation of two unrelated individuals. The presumption, therefore, is in favor of the view that the two results may be brought about in the same way, although we should be careful against a too ready acceptation of this plausible argument; for we have ample evidence to show that many closely similar (if not identical) responses of organisms may be brought about by very different agencies. The experiments seem to indicate that paramœcium might go on indefinitely reproducing by division, provided its environment is changed from time to time. If this is true, it is conceivable that the same thing is accomplished through conjugation. In the light of this possible interpretation much of the mystery connected with the term rejuvenescence is removed, for we see that there is nothing in the process itself except that it brings the organism into a new relation with other substances. Difficult as it assuredly is to understand how this benefits the animal, the experimental fact shows, nevertheless, that such a change is for its good. That there is really nothing in the process of conjugation itself apart from the difference in the constitution of the conjugating individuals is shown by the result that the union of individuals having the same history and kept under the same conditions is of no benefit.

Can we apply this same conception to the process of fertilization in the higher animals and plants? Is the substance of which their bodies are made of such a sort that it cannot go on living indefinitely under the same conditions, but must at times be supplied with a new environment? If this could be established, we could see the advantage of sexual reproduction over the non-sexual method. It would be extremely rash at present to make a generalization of this kind, for there are many forms known in which the only method of propagation that exists is the non-sexual one. In other words, there are no grounds for the assumption that this is a necessary condition for all kinds of protoplasm, but only for certain kinds.

In the insects, crustaceans, rotifers, and in some plants there are a few species whose egg develops without fertilization. This makes it appear probable that the particular kind of protoplasm of these animals does not absolutely require union from time to time with the protoplasm of another individual having a somewhat different constitution.