There is also an interesting parallel between the effects of solutions on the protozoans in Calkins’s experiments and certain results that have been obtained in artificial parthenogenesis. It has been stated, that by brushing the unfertilized eggs of the silkworm moth a larger percentage will develop parthenogenetically; and more recently it has been shown by Matthews that by agitation of the water in which the unfertilized eggs of the starfish have been placed many of them will begin their development. It was first shown by Richard Hertwig that by putting the unfertilized eggs of the sea-urchin in strychnine solutions, they will begin to segment, and I obtained the same results much better by placing the eggs in solutions of magnesium chloride. Loeb then succeeded in carrying the development to a later stage by using a different strength of the same solution, as well as by other solutions. Under the most favorable circumstances some of the eggs may produce larvæ that seem normal in all respects, but whether they can develop into adult sea-urchins has not yet been shown.
These results indicate that one at least of the factors of fertilization is the stimulus given to the egg. On the other hand, the lack of vigor shown by many eggs that have been artificially fertilized indicates that some other result is also accomplished by the normal method of fertilization that is here absent. This may mean no more than that as yet we have not found all the conditions necessary to supply the place of the spermatozoon.
In our study of the phenomena of adaptation we have found that sometimes the adaptation is for the benefit of the individual and at other times for the benefit of the species. May it not be true also that the process of sexual reproduction has more to do with a benefit conferred on the race rather than on the individual? In fact, Weismann has elaborated a view based on the conception that the process of sexual reproduction is beneficial to the race rather than to the individual. His idea, however, is not so much that the result is of direct benefit to a particular species, as it is advantageous to the formation of new species from the original one. In a sense this amounts, perhaps, to nearly the same thing, but in another sense the idea involves a somewhat different point of view.
According to his view “the deeper significance of conjugation” and of sexual reproduction is concerned “with the mingling of the hereditary tendencies of two individuals.” In this way, through the different combinations that are formed, variations which he supposes are indispensable for the action of natural selection originate. The purpose of the sexual process is solely, according to Weismann, to supply the variations for natural selection. If it be asked how this process has been acquired for the purpose of supplying natural selection with the material on which it can work, we find the following reply given by Weismann. “But if amphimixis [by which he means the union of sex-cells from different individuals] is not absolutely necessary, the rarity of purely parthenogenetic reproduction shows that it must have a widespread and deep significance. Its benefits are not to be sought in the single individual; for organisms can arise by agamic methods, without thereby suffering any loss of vital energy; amphimixis must rather be advantageous for the maintenance and modification of species. As soon as we admit that amphimixis confers some such benefits, it is clear that the latter must be augmented, as the method appears more frequently in the course of generations; hence we are led to inquire how nature can best have undertaken to give this amphimixis the widest possible range in the organic world.” Nature, Weismann says, could find no more effectual means of bringing about the union of the sexual cells than by rendering them incapable of developing alone. “The male germ-cells, being specially adapted for seeking and entering the ovum, are, as a rule, so ill provided with nutriment that their unaided development into an individual would be impossible; but with the ovum it is otherwise, and accordingly the ‘reduction division’ removes half the germ-plasm and the power of developing is withdrawn.” It can scarcely be claimed, in the light of more recent discoveries, that the reduction division takes place in order to prevent the development of the ovum, for how then could we explain the corresponding division of the male germ-cells?
Whatever means has been employed to bring about the process of sexual reproduction, the guiding principle is supposed by Weismann to be natural selection as stated in the following paragraph: “If we regard amphimixis as an adaptation of the highest importance, the phenomenon can be explained in a simple way. I only assume that amphimixis is of advantage in the phyletic development of life, and furthermore that it is beneficial in maintaining the level of adaptation, which has been once attained, in every single organism; for this is as dependent upon the continuous activity of natural selection as the coming of new species. According to the frequency with which amphimixis recurs in the life of a species, is the efficiency with which the species is maintained; since so much the more easily will it adapt itself to new conditions of life, and thus become modified.”
Thus we reach the somewhat startling conclusion that through natural selection the germ-cells and their protozoan prototypes have been rendered incapable, through natural selection, of reproducing by non-sexual methods, in order that variations may be supplied for the farther action of this same process of natural selection. The speculation has the appearance of arguing in a circle, although if it were worth the attempt an ingenious mind might perhaps succeed in showing that such a thing is not logically inconceivable.
It seems strange that a claim of this sort should have been made, when it is so apparent that the most immediate effect of intercrossing is to swamp all variations that depart from the average. Even if it were true that new combinations of characters would arise through the union of the germ-cells of two different animals, it is certainly true that in the case of fluctuating variations this new combination would be lost by later crossing with average individuals. Moreover, it is well known that variations occur amongst forms that are produced asexually. On the whole, it does not seem to be a satisfactory solution of the problem to assume that sexual reproduction has been acquired in order to supply natural selection with material on which it may work.
Our examination of the suggestions that have been made and of the speculation indulged in, as to what benefit the process of sexual reproduction confers on the animals and plants that make use of this method of propagation, has failed to show convincingly that any advantage to the individual or to the species is the outcome. This may mean, either that there is no advantage, or that we have as yet failed to understand the meaning of the phenomenon. The only light that has been thrown on the question is that a certain amount of renewed vigor is a consequence of this process, but we cannot explain how this takes place. There is also the suggestion that the union of different cells produces the same beneficial effect as a change in the conditions of life produces on the organism. The bad effects of close interbreeding that seem sometimes to follow is explicable on this view. This, it seems to me, is the most plausible solution of the question that has been advanced; but, even if this should prove to be the correct view, we need not assume that the process has been acquired on account of this advantage, for there is nothing to show that it has been acquired in this way.