interesting that I must briefly outline it to you. [16] There are different kinds of 'mixed forms' among ants, which may owe their origin to a variety of conditions, as Forel, Wasmann, and Emery have shown in detail. Like Forel and myself, Pater Wasmann had supposed that the reason of this kind of mixed form (the so-called pseudogynous worker) lay in an abnormality of the constitution of the germ-plasm, but he now regards it as the result of a change in the mode of rearing instituted by the workers with respect to the constitutionally female or queen larvæ, because there was a scarcity of workers. The hypothesis sounds very daring, but it is well founded, at least in so far that there really is a reason why a scarcity of females must occur at certain times in some colonies of ants, and this might certainly determine the workers in charge of the larvæ to feed females with worker food, so as to rear them to render the necessary assistance. This reason lies in the occasional presence of a parasitic beetle, _Lomechusa strumosa_, whose larvæ, curiously enough, are cared for and fed by the ants as though they were their own, and in return they eat up the larvæ of the ants, often destroying them in large numbers. Wasmann informs us that the parasitic larvæ grow up just at the time at which the ants are rearing their workers, and it is these, therefore, which fall victims to the Lomechusa-larvæ, and the result is that a scarcity of young workers must soon make itself felt. The workers seek to make this good by rearing as workers all the larvæ previously destined for queens. But this only succeeds partially, because the development towards true females has already begun; thus mixed forms arise. 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.