Fig. 106. Three workers of the same species of Indian Ant (Pheidologeton diversus), drawn from specimens supplied by Prof. August Forel. A, the largest, B, the intermediate, C, the smallest form.
Where, then, shall we look for this other factor, if not in the processes of selection, in the selecting of the most suitable variations among all those which occur? We are confronted with the alternative of either working out a sufficient explanation with this factor, or of giving up the attempt at explanation altogether. Yet the application of the principle of selection in relation to the neuters of colony-forming insects is by no means simple, for, as the workers are sterile, a modification of them through processes of breeding cannot begin directly with themselves. The workers which exhibit the most suitable variations cannot be selected for breeding, but only their parents, the sexual animals, and these according to whether they produce better workers or worse. This is how Darwin looked at the matter, and his view receives support from one peculiarity in the composition of these animal colonies, whose significance becomes apparent in relation to this problem. It has long been known that in a bee-hive there are from 10,000 to 20,000 workers, but only one true female, the so-called queen, and the meaning of this remarkable arrangement probably is, that the adaptation of the workers through natural selection becomes much more easily possible, since the whole number are the children of a single pair. It is not the individual workers, but the whole colony, that is, the whole progeny of the queen, which is selected, according to the greater or less degree of effectiveness displayed by the workers. Strictly speaking, it is the single queen that is selected in relation to her power of producing superior or inferior workers. A colony whose queen was unsatisfactory in this respect could not hold its own in the struggle for existence, and only the best colonies and the best hives would survive, that is, through their descendants. If the hive contained a hundred queens instead of a single queen the process of selection would be much more complex and less clear, and it is even quite conceivable that the production of specially modified workers, adapted to their functions, or of two or three different kinds of workers, would not have been possible at all. For it would not have helped much if one out of a hundred females had produced workers of better structure; only a majority of such females could give the colony any advantage as compared with other colonies.
It has not been definitely established whether, among ants, a single female is in all cases the founder of the whole colony, but it is certain that there are only a few females. In the tropical Termites we know that the ovaries of the female attain to such a colossal size that one female must certainly suffice for the necessities of the largest colonies. Grassi has shown, indeed, that, as far as the South European Termites are concerned, not only are there several females present, but that even the workers frequently reproduce; but the Termites in general are inhabitants of warm countries, and the few European species probably hardly represent the original composition of these animal colonies. But of the tropical species, which have as yet not been sufficiently studied, we know at least the extraordinary dimensions of the body, and the corresponding fertility of the queens, and we conclude from this that only a few can be present in each termitary[18].
[18] Ingwe Sjostedt has recently established in Africa that it is usually a single queen and a single king that found a termitary (Schwed. Akad. Abh. Bd. 34, 1902).
Now that we have discussed all these facts it will not be out of place to summarize the results, in as far as they have any relation to the acceptance or rejection of the theory of the inheritance of acquired characters.
No direct proof of such transmission could be found; on the contrary, it has been shown that all that has hitherto been advanced as such will not stand the test of close examination; an inheritance of wounds and mutilations does not exist, the transmission of traumatically induced epilepsy is not only doubtful as regards its causes, but cannot even be considered as the transmission of a particular morphological lesion.
We may regard as indirect proofs such facts as can only be explained on the assumption of this mode of inheritance, and in this connexion our opponents have cited especially the correspondence between modifications acquired through use in the individual lifetime, and worked out through histonal selection, with the phyletic transformations of the same parts. But it has been shown that a number of parts which do not function actively at all, but only passively, and thus cannot be caused to change through use, like the hard skeletal parts of the Arthropods, vary phyletically in the same certain and direct course as those which function actively, so that we have every ground for assuming that there are other factors operating in the transformation of the active as well as of the passive parts. Finally, we discussed the last and strongest argument which has been put forward in favour of the Lamarckian principle, that of co-adaptation, that is, the simultaneous adaptation of many parts co-operating in a common action, and we were able to controvert this altogether by showing that exactly similar phenomena of co-adaptation occur in systems of passively functioning parts, and further, that they occur also among the workers of ants and bees, that is, in animals which do not reproduce, and which, therefore, cannot transmit the acquired results of exercise during their life.
We therefore reject—and are compelled to reject—the Lamarckian principle, not only on the ground that it cannot be proved correct, but also because the phenomena, to explain which it is used, occur also under circumstances which absolutely exclude any possibility of the co-operation of this principle.
Supplementary note on the Transmissibility of Acquired
Functional Modifications.
I cannot conclude this section without some reference to the utterances of some naturalists who have quite recently attempted to represent the inheritance of functional modifications as a conceivable and even a necessary assumption.