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.

Passing by other invertebrate groups made up of species arranged like
higher animals in smaller and larger branches according to their degree of
fundamental similarity, we arrive at a place in the scale occupied by
two-layer animals without the highly developed and clearly differentiated
organic systems of the forms above. The fresh-water animal _Hydra_
exemplifies the creatures of this level, where also we find sea-anemones
and the soft polyps which form corals and coral reefs by their combined
skeletons. _Hydra_ is an animal to which we must return again and again as
we study one or another aspect of organic evolution. In general form it is
a