Butterflies, beetles, bees, and grasshoppers seem at first sight to be entirely different, even though they agree in being more or less segmented. But all of them have heads with four pairs of appendages of the same essential plan, middle thoracic regions of three segments more or less united, bearing three pairs of legs and usually two pairs of wings, while the hinder part is a freely jointed abdomen without real limbs. In these respects the countless varieties of insects agree so that they also like crustacea of various kinds seem to have been derived from wormlike animals with more simply segmented bodies. Indeed spiders and scorpions and their relatives of the group arachnida prove for similar reasons to be derivatives of the same original stock, and own cousins of the insects. In nearly every one of the invertebrate branches we find representatives which interest us chiefly because they appear to have reached their present condition by retrograde evolution. Barnacles are really crustacea, but they have lost their eyes as well as some other structures that are most useful in animals with a free existence, because they have adopted a fixed mode of life, which has also brought about the loss of the original freely jointed character of the body. A tapeworm as an example of internal parasites is an extremely degenerate form which lacks a digestive tract, because this is superfluous in an animal which lives bathed in the nutrient fluids of its host. Comparing it in other respects with other low wormlike creatures, it appears to be a relative of peculiar simple worms with complete organization and independence of life. All these degenerate forms enlarge our conception of adaptation by adding the essential point that progress is not always the result of evolution. Indeed we have learned this in the case of vestigial and rudimentary structures of higher forms like whales, and now we find that entire animals may degenerate as a result of changes no less adaptive than progressive modifications.