heless it fails to explain the peculiar condition presented by
_Syllis_ and some other annelids, where a new head is formed at intervals
in certain segments of the body. Here there is evidently an innate tendency
to the development at intervals of a complex whole. It is not the budding
out or spontaneous fission of certain segments, but the transformation in a
definite and very peculiar manner of parts which already exist into other
and more complex parts. Again, the processes of development presented by
some of these creatures do not by any means point to an origin through{170}
the linear coalescence of primitively distinct animals by means of
imperfect segmentation. Thus in certain Diptera (two winged flies) the
legs, wings, eyes, &c., are derived from masses of formative tissue (termed
imaginal disks), which by their mutual approximation together build up
parts of the head and body,[170] recalling to mind the development of
Echinoderms.

[Illustration: AN ANNELID DIVIDING SPONTANEOUSLY.
(A new head having been formed towards the hinder end of the body of the
parent.)]

Again, Nicholas Wagner found in certain other Diptera, the Hessian flies,
that the larva gives rise to secondary larvae within it, which develop and
burst the body of the primary larva. The secondary larvae give rise,
similarly, to another set within them, and these again to another[171] set.

Again, the fact that in _Taenia echinococcus_ one egg produces numerous
individuals, tends to invalidate the argument that the increase of segments
during development is a relic of specific genesis.

Mr. H. Spencer seems to deny serial homology to the mollusca, but it is
difficult to see why the shell segments of chiton are not such homologues
because the segmentation is superficial. Similarly the external processes
of eolis, doris, &c., are good examples of serial homology, as also are
plainly the successive chambers of the orthoceratidae. Nor are parts of a
series less serial, because arranged spirally, as in most gasteropods. Mr.
Spencer observes of the molluscous as of the vertebrate animal, "You cannot
cut it into transverse slices, each of which contains a digestive organ, a
respiratory organ, a reproductive organ, &c."[172] But the same may be said
of every single arthropod and annelid if it be meant that all these organs
are not contained in every possible slice. While if it be meant that parts
of all such organs are contained in certain slices, then so