are broken down or reabsorbed, just as in the human embryo the gillarches disappear, or are transformed into organs required in the next higher stage.
If, in the insect pupal stage, the development of the higher structural stage begins simultaneously with, or immediately after, that of a lower stage, the latter is interfered with by a superior energy. It can not obtain full unfoldment, and may gain but a rudimentary formation, which may be immediately reabsorbed, to answer the demands of the superior modifying energy. Two unlike energies seem fighting for the nutriment, and the stronger wins. Instances of this principle of development are common in embryo growth, and act to check or to completely abolish the unfoldment of ancestral features. In the case of the ant we may look upon it as the cause of the lack of appearance of the worker characteristics in the development of the queen, and of their full development where the conditions are such as to prevent the innate powers of sexual unfoldment from coming into play, and to restrict development at a lower level. It need scarcely be added that in the case of these insects the check to development is final. On leaving the pupa-case, they enter upon a life of active nutrition, in which the powers of development already in operation may produce their full results, but in which the latent higher powers are definitely restrained. In all cases of insect development, and doubtless to a certain extent in all animals, a state of passivity is requisite to active transformation of tissue, while simple growth is the prevailing tendency in states of activity and abundant nutrition. In these latter states organic development may proceed, but it is simply the completion of lines of development which began in the passive state. New lines of development do not begin during nutritive activity. Of this principle many illustrations might be given, had we the space here to adduce them.
In the case already cited, of the hydroid polyps, this principle of development yields some remarkable results. In many instances the sexual individual; unfold into the full medusoid type, and leave the colony to enjoy a free life. In others they remain attached to the colony, and are more or less checked in their full development. This check to development is so great, in certain instances, that a mere bud appears, to bear the generative products. Thus the sexual, instead of being the typical form, remains as a formless protrusion of the polyp stem, or the germs may originate in this stem with no form development.
Yet this seeming anomaly is not without its explanation under the above principle. Where free Medusæ are produced, the sexual organs and products do not appear until after full development and freedom from the colony are attained. In the other cases mentioned the generative products appear earlier, and it is probably their appearance that checks further form development. The innate tendency to de-
