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, wings, eyes, &c., respectively; and these severally formed parts grow together, and build up the head and body by their mutual approximation.