96 THE EVOLUTION THEORY 



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 Lamarck ian 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 wliich 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 lead to a dimorphism of the workers. 

 In many species, certainly, the onXy difterence 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 megalocephola, which is abundant in Italy, the 

 laro-er 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 Golohopsis 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 [CEcodoma 

 cepkalotes) Bates described three diff"erent types of worker, differing in 

 size, and although he was not able to determine Avith 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 adapta- 

 tions to the differences in their functions. The same is true of the 

 Indian ant, Pheidologeton diversus, depicted in Fig. io6, 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 



