From all the facts we have discussed we can confidently conclude that the differences in structure, which distinguish the workers from the true females, do not depend upon the influence, in the individual lifetime, of a poorer diet, but upon variation in the primary constituents of the germ; we must conceive of the germ-plasm of ants as containing, in addition to male and female ids, special ids of workers, in which the determinants of wing and ovary are degenerate in some degree, while the determinants of other parts, such as the brain, are more highly developed. The manner of feeding, however, and perhaps the mingling with the food of a special secretion of the salivary glands, acts as a stimulus which determines whether one kind of id or another is to be liberated, that is, to become active and to enter on the path of development. A proof of this view is to be found, it seems to me, in the existence of transition forms between workers and true females, which was first brought to general knowledge by Forel. Perhaps it would be better to call these 'mongrel forms' for their various parts do not maintain a medium between the two types, but many parts follow the type of the worker, and others that of the true female. Thus Forel twice found a nest of the red wood-ant which contained a large number of these mixed forms, all of which possessed the small head and large curved thorax of the queen, but otherwise resembled the workers in size and appearance, and also in the degeneration of the ovaries. Many of them were very small, only 5 mm. in length, and had probably received very little food, and, according to the theory of direct influence, these should have been pure workers. That they possessed the head and thorax of a queen is a proof that the characters of both forms of individual were present in the germ-plasm as primary constituents, or indeed entire ids. In normal circumstances only one kind of these ids would have become active, either the worker-id or the queen-id, but in abnormal circumstances they might both be liberated to activity simultaneously, and then they would stamp one part of the body with the character of a queen, another with that of a worker. Forel observed one of these nests in two successive years, and both times found the mixed forms in large numbers[16]. In the second year he found a great number of newly-emerged individuals of this type. I have already inferred from this observation that the mixed forms were probably in both years the offspring of the same mother, and this may well have been the case. My further conclusion, that the mixed forms must be due to some abnormality in the constitution of the germ-plasm of the maternal eggs, no longer appears to me so convincing as it did formerly, because, in the interval, we have learnt, through that indefatigable investigator of ants, Pater Wasmann, that there is another possible explanation of these mixed forms; it, too, is based upon a hypothesis, but it is so