ve or analogical characters, though these often govern the whole
economy of the individual, but depending on any character which varies
least, and especially on the forms through which the embryo passes, and,
as was afterwards shown, on the presence of rudimentary and useless
organs. The alliance between the nearest species in _distinct_ groups
being general and not especial; the close similarity in the rules and
objects in classifying domestic races and true species. All these facts
were shown to follow on the natural system being a genealogical system.

In the eighth chapter, the unity of structure throughout large groups,
in species adapted to the most different lives, and the wonderful
metamorphosis (used metaphorically by naturalists) of one part or organ
into another, were shown to follow simply on new species being produced
by the selection and inheritance of successive _small_ changes of
structure. The unity of type is wonderfully manifested by the similarity
of structure, during the embryonic period, in the species of entire
classes. To explain this it was shown that the different races of our
domestic animals differ less, during their young state, than when full
grown; and consequently, if species are produced like races, the same
fact, on a greater scale, might have been expected to hold good with
them. This remarkable law of nature was attempted to be explained
through establishing, by sundry facts, that slight variations originally
appear during all periods of life, and that when inherited they tend to
appear at the corresponding period of life; according to these
principles, in several species descended from the same parent-stock,
their embryos would almost necessarily much more closely resemble each
other than they would in their adult state. The importance of these
embryonic resemblances, in making out a natural or genealogical
classification, thus becomes at once obvious. The occasional greater
simplicity of structure in the mature animal than in the embryo; the
gradation in complexity of the species in the great classes; the
adaptation of the larvae of animals to independent powers of existence;
the immense difference in certain animals in their larval and mature
states, were all shown on the above principles to present no difficulty.

In the <ninth> chapter, the frequent and almost general presence of
organs and parts, called by naturalists abortive or rudimentary, which,
though formed with exquisite