The Evolution Theory, Vol. 2 of 2

by August Weismann

This explanation would be rather in the air if we did not know that, among bees, such changes in the manner of rearing are by no means uncommon. Indeed they occur regularly when the queen of a hive perishes and no more 'female' eggs are in store; young worker larvæ are then fed with royal food, and these develop into queens. There can thus be no doubt that these insects have it in their power to liberate to activity either the female ids or the worker ids by a specific mode of feeding, and there is nothing contrary to reason in admitting the possibility of an alternation of this influence in the course of development, for something analogous occurs in regard to secondary sexual characters, as, for instance, the appearance of male decorative colours in ducks that have become sterile.

But this change in the mode of rearing bee-larvæ gives rise to pure queens and not to mixed forms, and we must therefore regard it as undecided whether Wasmann's explanation is correct in this case, and whether an abnormality in the constitution of the germ-plasm may not be the true cause of this or other kinds of mixed forms among ants. In any case the 'Lomechusa hypothesis' rests upon the assumption of different kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and the differences between the worker and queen-ants have their cause in this, and not directly in the kind of larval food.

If there were not different ids corresponding to the different kinds of individuals in the germ-plasm a kind of polymorphism might indeed have arisen in the colony through differences in nutrition, but it could not have been of the kind we now see—that is, a sharply defined differentiation of persons, in adaptation to their different functions. This presupposes elements in the germ which can vary slowly and consistently in a definite direction without causing any change in the rest of the germ.

This state of affairs gives to the phyletic evolution of the workers a great theoretical significance, for it proves that positive as well as negative variations of the most diverse parts of the body, that simultaneous and correlative variations of many parts, can take place in the course of the phylogeny, without the co-operation of the Lamarckian factor. I have not hitherto laid any special emphasis upon the degree of differences occurring between workers and queens; but I must now add that this may far exceed the degree that we are familiar with in our common indigenous ants, both in regard to instinct and to bodily form. Even in the red Amazon ant of Western Switzerland, Polyergus rufescens, we find quite a new instinct[17], that of carrying off the pupæ of other species of ants, not to devour, but to introduce them to their own nest and thus secure 'slaves.' For these workers of a strange species, which emerge in a strange nest, naturally regard the place of their birth as their home, and do there what instinct impels them, and what they would have done in the nest of their parents: they feed the larvæ, fetch food, collect building material, and so on. The domestic activity of the workers of the master-species thus becomes superfluous, and they have ceased to exercise it, and have now entirely lost the power of caring for their brood, searching for food, and keeping up the nest. They have even forgotten how to take food themselves, because they are always fed by the 'slaves.' Forel informs us—and I have myself repeated the experiment—that Polyergus workers, which are shut up with a drop of honey on the floor of their prison, will leave it, their favourite food, untouched, and finally starve, unless one of their 'slaves' be shut up with them. As soon as this happens, and the slave perceives the honey, it partakes of it, and then the 'mistress' comes and strokes the 'slave' with her antennæ to signify her desires, whereupon the 'slave' proceeds to feed her from its own crop.

[17] 'New' in this sense, that the instinct is not exhibited by most worker-ants, that it did not occur in the primaeval ancestors of modern ants. It is, however, exhibited by a number of modern forms, and even by some German species.

But while the Polyergus workers have forgotten their domestic habits, and have even ceased to be able to recognize their food, remarkable changes have taken place in their jaws; these have lost the blunt teeth on the inner margin, which, in other species, serve for masticating the food, for seizing building material, and for other domestic occupations, and have become sharp weapons, bent in the form of a sabre, very well suited for piercing the head of an enemy, but also well adapted for carrying off the pupæ, because they can seize them without doing them any injury.

No one will doubt that the predatory expeditions of the Amazon ants, and the slave-making habit, can only have developed after the habit of living in large companies had long existed, and this case proves that variations of instinct, as well as of bodily structure, can take place even after the workers have long been sterile. The case is the more instructive that it seems as if it were due to the transmission of a newly acquired and inherited habit of life, while in point of fact these Amazon-workers can transmit nothing, because they bear no offspring. But if old instincts can be lost, and new ones acquired, when all possibility of inheritance is excluded, we see that Nature has no need of the Lamarckian factor of modification for her transformations and new adaptations.

If we wish to understand clearly that, in these changes, we have to do not merely with the alteration of a single part, but of many parts which all work together, we have only to think of the still more striking physical modifications which have taken place in many tropical ants, and which have led to a dimorphism of the workers. In many species, certainly, the only difference is in size, so that one can distinguish between large workers and small, and the former are sometimes five times as big as the latter. But even in the South European Pheidole megalocephala, which is abundant in Italy, the larger workers are also different in structure from the smaller, for they have an enormous head with powerful jaws. They are usually known as 'soldiers,' and are entrusted with the defence of the colony. Emery directly observed in regard to Colobopsis truncata, an ant which lives in the trunks of trees, that the soldiers, with their enormous heads, occupied all the entrances to the nest, ready to seize any intruder with their powerful jaws. In the Sauba ant (Œcodoma cephalotes) Bates described three different types of worker, differing in size, and although he was not able to determine with certainty what the particular function of each was, there can be no doubt that they have special offices, and that the differences in their structure are adaptations to the differences in their functions. The same is true of the Indian ant, Pheidologeton diversus, depicted in Fig. 106, whose three forms of workers I owe to the kindness of Professor August Forel.

If the increase in the size of the head and jaws must bring with it an increase in the thickness of the skeleton of these parts, as well as a strengthening of the musculature of the head, it follows that the strain on the body must be greater, just as in the case of the increase in the weight of the stag's antlers, so that the skeleton of the thorax must likewise have become thicker and heavier, the muscles and nerves of the legs stronger, the articulations of the joints capable of greater resistance; in short, a whole series of variations of other parts must have taken place simultaneously, if the primary variation was to be of use, and not to lead to the destruction of its possessor. Here again we have a proof that the co-adaptation of many parts can take place without any intervention of the Lamarckian principle, and that there must be some other factor which brings this about.