Under this point of view,
the productions of Great Britain stand much higher in the scale than those
of New Zealand. Yet the most skilful naturalist, from an examination of
the species of the two countries, could not have foreseen this result.
Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of the
former and less modified condition of the species. This view may be true,
and yet may never be capable of proof. Seeing, for instance, that the
oldest known mammals, reptiles, and fishes strictly belong to their proper
classes, though some of these old forms are in a slight degree less
distinct from each other than are the typical members of the same groups at
the present day, it would be vain to look for animals having the common
embryological character of the Vertebrata, until beds rich in fossils are
discovered far beneath the lowest Cambrian strata--a discovery of which the
chance is small.
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