The long argument of Fleischmann in regard to the origin of the fresh-water snails, as illustrated by the planorbis series, and also the origin of the nautiloid group, has been recently dealt with fully by Plate, and, therefore, need not be considered here.
With fine scorn Fleischmann points to the crudity of the ideas of Oken and of Haeckel in regard to the embryology (or the ontogeny) repeating the ancestral history (or the phylogeny). We may consider briefly (since we devote the next chapter almost entirely to the same topic) the exceptions to this supposed recapitulation, which Fleischmann has brought together. The young of beetles, flies, and butterflies creep out of the egg as small worm-like forms of apparently simple organization. They have a long body, composed of a series of rings; the head is small and lacks the feelers, and often the faceted eyes. The wings are absent, and the legs are short. At first sight the larva appears to resemble a worm, and this led Oken to conclude that the insects appear first in the form of their ancestors, the segmented worms. If we examine the structure of the larva more carefully, we shall find that there are a great many differences between it and the segmented worms; and that even the youngest larva is indeed a typical insect. The tracheæ, so characteristic of the group of insects, are present, the structure of the digestive tract with its Malpighian tubes, the form of the heart, the structure of the head, as well as the blastema of the reproductive organs, show in the youngest larva the type of the insects. In other words the body of the caterpillar is formed on exactly the same fundamental plan as that of the butterfly.
In regard to the larval forms of other groups we find the same relations, as, for example, in the amphibians. The young of salamanders, toads, and frogs leave the egg not in the completed form, but as small tadpoles adapted to life in the water. A certain resemblance to fish cannot be denied. They possess a broad tail, gills (rich in blood vessels) on each side of the neck, and limbs are absent for a long time. These are characters similar to those of fish, but a more careful anatomical examination destroys the apparent resemblance. The superficial resemblances are due to adaptation to the same external conditions.