me of the
mollusca may also be included.

Another objection to Mr. Spencer's speculation is derived from
considerations which have already been stated, as to past time. For if{171}
the annulose animals have been formed by aggregation, we ought to find this
process much less perfect in the oldest form. But a complete development,
such as already obtains in the lobster, &c., was reached by the Eurypterida
and Trilobites of the palaeozoic strata; and annelids, probably formed
mainly like those of the present day, abounded during the deposition of the
oldest fossiliferous rocks.

[Illustration: TRILOBITE.]

Thirdly, and lastly, as regards such serial homology as is exemplified by
the backbone of man, there are also several objections to Mr. Spencer's
mechanical explanation.

On the theory of evolution most in favour, the first Vertebrata were
aquatic. Now, as natation is generally effected by repeated and vigorous
lateral flexions of the body, we ought to find the segmentation much more
complete laterally than on the dorsal and ventral aspects of the spinal
column. Nevertheless, in those species which, taken together, constitute a
series of more and more distinctly segmented forms, the segmentation
gradually increases _all round_ the central part of the spinal column.

Mr. Spencer[173] thinks it probable that the sturgeon has retained the
notochordal (that is, the primitive, unsegmented) structure because it{172}
is sluggish. But Dr. Guenther informs me that the sluggishness of the common
tope (_Galeus vulgaris_) is much like that of the sturgeon, and yet the
bodies of its vertebrae are distinct and well-ossified. Moreover, the great
salamander of Japan is much more inert and sluggish than either, and yet it
has a well-developed, bony spine.

I can learn nothing of the habits of the sharks _Hexanchus_, _Heptanchus_,
and _Echinorhinus_, but Mueller describes them as possessing a persistent
_chorda dorsalis_.[174] It may be they have the habits of the tope, but
other sharks are amongst the very swiftest and most active of fishes.

In the bony pike (_lepidosteus_), the rigidity of the bony scales by which
it is completely enclosed must prevent any excessive flexion of the body,
and yet its vertebral column presents a degree of ossification and
vertebral completeness greater than that found in any other fish whatever.

Mr. Spencer supports his argument by the non-segmentation of the anterior
end of the skeletal axis, _i