omes largely into play even in the case of
continuous growth, as we shall see when we consider the inheritance of
modifications at corresponding ages.

We are led to the same conclusion, namely, the independence of parts
successively developed, by another and quite distinct group of facts. It is
well known that many animals belonging to the same class, and therefore not
differing widely from each other, pass through an extremely different
course of development. Thus certain beetles, not in any way remarkably
different from others of the same order, undergo what has been called a
hyper-metamorphosis--that is, they pass through an early stage wholly
different from the ordinary grub-like larva. In the same sub-order of
crabs, namely, the Macroura, as Fritz {368} Mueller remarks, the river
cray-fish is hatched under the same form which it ever afterwards retains;
the young lobster has divided legs, like a Mysis; the Palaemon appears
under the form of a Zoea, and Peneus under the Nauplius-form; and how
wonderfully these larval forms differ from each other, is known to every
naturalist.[889] Some other crustaceans, as the same author observes, start
from the same point and arrive at nearly the same end, but in the middle of
their development are widely different from each other. Still more striking
cases could be given with respect to the Echinodermata. With the Medusae or
jelly-fishes Professor Allman observes, "the classification of the Hydroida
would be a comparatively simple task if, as has been erroneously asserted,
generically-identical medusoids always arose from generically-identical
polypoids; and on the other hand, that generically-identical polypoids
always gave origin to generically-identical medusoids." So, again, Dr.
Strethill Wright remarks, "in the life-history of the Hydroidae any phase,
planuloid, polypoid, or medusoid, may be absent."[890]

According to the belief now generally accepted by our best naturalists, all
the members of the same order or class, the Macrourous crustaceans for
instance, are descended from a common progenitor. During their descent they
have diverged much in structure, but have retained much in common; and this
divergence and retention of character has been effected, though they have
passed and still pass through marvellously different metamorphoses. This
fact well illustrates how independent each structure must be from that
which precedes and follows it in the course of development.