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. [Illustration: 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.]