hers, that they all receive a common name--the
ribs. There also (_i.e._ in the skeleton) we find a still more remarkable
series of similar parts, the joints of the spine or backbone (vertebrae),
which are admitted by all to possess a certain community of structure.{163}

It is in their limbs, however, that the Vertebrata present the most obvious
and striking serial homology--almost the only serial homology noticeable
externally.

The facts of serial homology seem hardly to have excited the amount of
interest they certainly merit.

Very many writers, indeed, have occupied themselves with investigations and
speculations as to what portions of the leg and foot answer to what parts
of the arm and hand, a question which has only recently received a more or
less satisfactory solution through the successive concordant efforts of
Professor Humphry,[163] Professor Huxley,[164] the Author of this
work,[165] and Professor Flower.[166] Very few writers, however, have
devoted much time or thought to the question of serial homology in general.
Mr. Herbert Spencer, indeed, in his very interesting "First Principles of
Biology," has given forth ideas on this subject, which are well worthy
careful perusal and consideration, and some of which apply also to the
other kinds of homology mentioned above. He would explain the serial
homologies of such creatures as the lobster and centipede thus: Animals of
a very low grade propagate themselves by spontaneous fission. If certain
creatures found benefit from this process of division remaining incomplete,
such creatures (on the theory of "Natural Selection") would transmit their
selected tendency to such incomplete division to their posterity. In this
way, it is conceivable, that animals might arise in the form of long chains
of similar segments, each of which chains would consist of a number of
imperfectly separated individuals, and be equivalent to a series of
separate individuals belonging to kinds in which the fission was complete.
In other words, Mr. Spencer would explain it as the coalescence of {164}
organisms of a lower degree of aggregation in one longitudinal series,
through survival of the fittest aggregations. This may be so. It is
certainly an ingenious speculation, but facts have not yet been brought
forward which demonstrate it. Had they been so, this kind of serial
homology might be termed "homogenetic."

The other kind of serial repetitions, namely, those of the vertebral
c