community of organisation I found the artifice of an archetype
vertebrate animal essential; and from the demonstration of its
principle, which I then satisfied myself was associated with and
dominated by that of "adaptation to purpose," the step was inevitable to
the conception of the operation of a secondary cause of the entire
series of species, such cause being the servant of predetermining
intelligent will.

But besides "derivation" or "filiation" another principle influencing
organisation became recognisable, to which I gave the name of
"irrelative repetition," or "vegetative repetition." The demonstrated
constitution of the vertebrate endoskeleton as a series of essentially
similar segments appeared to me to illustrate the law of irrelative
repetition.

These results of inductive research swayed me in rejecting direct or
miraculous creation, and in recognising a "natural law or secondary
cause" as operative in the production of species "in orderly succession
and progression."

_II.--Succession of Species, Broken or Linked?_

To the hypothesis that existing are modifications of extinct species,
Cuvier replied that traces of modification were due from the fossil
world. "You ought," he said, "to be able to show the intermediate forms
between the palaeotherium and existing hoofed quadrupeds."

The progress of palaeontology since 1830 has brought to light many
missing links unknown to the founder of the science. The discovery of
the remains of the hipparion supplied one of the links required by
Cuvier, and it is significant that the remains of such three-toed horses
are found only in deposits of that tertiary period which intervene
between the older palaeotherian one and the newer strata in which the
modern horse first appears to have lost its lateral hooflets.

The molar series of the horse includes six large complex grinders
individually recognisable by developmental characters. The
representative of the first premolar is minute and soon shod. Its
homologue in palaeotherium is functionally developed and retained, that
type-dentition being adhered to. In hipparion this tooth is smaller than
in palaeotherium, but functional and permanent. The transitory and
singularly small and simple denticle in the horse exemplifies the
rudiment of an ancestral structure in the same degree as do the hoofless
splint-bones; just as the spurious hoofs dangling therefrom in hipparion
are retained rudiments of the functionally de