s of structure; to deny this seems to do no less than to
affirm a startling paradox.

Mr. Darwin explains the appearance of some structures, the utility of which
is not apparent, by the existence of certain "laws of correlation." By
these he means that certain parts or organs of the body are so related to
other organs or parts, that when the first are modified by the action of
"Natural Selection," or what not, the second are simultaneously affected,
and increase proportionally or possibly so decrease. Examples of such are
the hair and teeth in the naked Turkish dog, the general deafness of white
cats with blue eyes, the relation between the presence of more or less down
on young birds when first hatched, and the future colour of their
plumage,[36] with many others. But the idea that the modification of any
internal or external part of the body of an Echinus carries with it the
effect of producing elongated, flexible, triradiate, snapping processes,
is, to say the very least, fully as obscure and mysterious as what is here
contended for, viz. the efficient presence of an unknown internal natural
law or laws conditioning the evolution of new specific forms from preceding
ones, modified by the action of surrounding conditions, by "Natural
Selection" and by other controlling influences.

The same difficulty seems to present itself in other examples of
exceptional structure and action. In the same Echinus, as in many allied
forms, and also in some more or less remote ones, a very peculiar mode of
development exists. The adult is not formed from the egg directly, but {46}
the egg gives rise to a creature which swims freely about, feeds, and is
even somewhat complexly organized. Soon a small lump appears on one side of
its stomach; this enlarges, and, having established a communication with
the exterior, envelopes and appropriates the creature's stomach, with which
it swims away and develops into the complete adult form, while the
dispossessed individual perishes.

Again, certain flies present a mode of development equally bizarre, though
quite different. In these flies, the grub is, as usual, produced from the
ovum, but this grub, instead of growing up into the adult in the ordinary
way, undergoes a sort of liquefaction of a great part of its body, while
certain patches of formative tissue, which are attached to the ramifying
air tubes, or tracheae (and which patches bear the name of "imaginal
disks"), give rise to the legs,