At the time of hatching the Cockroach resembles its parent in all essentials, the wings being the only organs which are developed subsequently, not as entirely new parts, but as extensions of the lateral edges of the thoracic terga. The mode of life of the young Cockroach is like that of the adult, and development may be said to be direct, or with only a trifling amount of metamorphosis. In the Thysanura even this small post-embryonic change ceases to appear, and the Insect, when it leaves the egg, differs from its parent only in size. It is probable that development without metamorphosis was once the rule among Insects. At present such is by no means the case. Insects furnish the most familiar and striking, though, as will appear by-and-by, not the most typical examples of development with metamorphosis. In many text-books the quiescent pupa and the winged imago are not unnaturally described as normal stages, which are exceptionally wanting in Orthoptera, Hemiptera, Thysanura, and other “ametabolous” Insects. It is, however, really the “holometabolous” Insects undergoing what is called “complete metamorphosis,” which are exceptional, deviating not only from such little-specialised orders as Thysanura and Orthoptera, but from nearly all animals which exhibit a marked degree of metamorphosis. We shall endeavour to make good this statement, and to show that the Cockroach is normal in its absence of conspicuous post-embryonic change, while the Butterfly, Bee, Beetle, and Gnat are peculiar even among metamorphic animals. _Animal Metamorphoses._ To investigate the causes of metamorphosis, let us select from the same sub-kingdom two animals as unlike as possible with respect to the amount of post-embryonic change to which they are subject. We can find no better examples than Amphioxus and the Chick. The newly-hatched Amphioxus is a small, two-layered, hollow sac, which moves through the sea by the play of cilia which project everywhere from its outer surface. It is a Gastræa, a little simpler than the Hydra, and far simpler than a Jelly-fish. As yet it possesses no nervous system, heart, respiratory organs, or skeleton. The most expert zoologist, ignorant of its life-history, could not determine its zoological position. He would most likely guess that it would turn either into a polyp or a worm. The Chick, on the other hand, at the tenth day of incubation, is already a Bird, with feathers, wings, and beak. When it chips the shell it is a young fowl. It has the skull, the skeleton, the toes, and the bill characteristic of its kind, and no child would hesitate to call it a young Bird. Amphioxus is, therefore, a Vertebrate (if for shortness we may so name a creature without vertebræ, brain, or skull), which develops with metamorphosis, being at first altogether unlike its parent. The Chick is a Vertebrate which develops directly, without metamorphosis. Let us now ask what other peculiarities go with this difference in mode of development.