their margins. We can
convince ourselves of this by actual sight when such composite capsules
fall apart after becoming ripe, because then every part displays an
opened pod."(1)

The theory thus elaborated of the metamorphosis of parts was presently
given greater generality through extension to the animal kingdom, in
the doctrine which Goethe and Oken advanced independently, that the
vertebrate skull is essentially a modified and developed vertebra. These
were conceptions worthy of a poet--impossible, indeed, for any mind that
had not the poetic faculty of correlation. But in this case the poet's
vision was prophetic of a future view of the most prosaic science.
The doctrine of metamorphosis of parts soon came to be regarded as of
fundamental importance.

But the doctrine had implications that few of its early advocates
realized. If all the parts of a flower--sepal, petal, stamen,
pistil, with their countless deviations of contour and color--are
but modifications of the leaf, such modification implies a marvellous
differentiation and development. To assert that a stamen is a
metamorphosed leaf means, if it means anything, that in the long sweep
of time the leaf has by slow or sudden gradations changed its character
through successive generations, until the offspring, so to speak, of a
true leaf has become a stamen. But if such a metamorphosis as this
is possible--if the seemingly wide gap between leaf and stamen may
be spanned by the modification of a line of organisms--where does the
possibility of modification of organic type find its bounds? Why may
not the modification of parts go on along devious lines until the remote
descendants of an organism are utterly unlike that organism? Why may we
not thus account for the development of various species of beings all
sprung from one parent stock? That, too, is a poet's dream; but is it
only a dream? Goethe thought not. Out of his studies of metamorphosis of
parts there grew in his mind the belief that the multitudinous species
of plants and animals about us have been evolved from fewer and fewer
earlier parent types, like twigs of a giant tree drawing their nurture
from the same primal root. It was a bold and revolutionary thought, and
the world regarded it as but the vagary of a poet.

ERASMUS DARWIN

Just at the time when this thought was taking form in Goethe's brain,
the same idea was germinating in the mind of another philosopher, an
Englishman of internationa