the principle of useful variations and their natural selection: and my
fundamental conception is this, that a great plan of development lies
at the foundation of the origin of the whole organic world, impelling
the simpler forms to more and more complex developments. How this law
operates, what influences determine the development of the eggs and
germs, and impel them to assume constantly new forms, I naturally
cannot pretend to say; but I can at least adduce the great analogy of
the alternation of generations. If a 'Bipinnaria', a 'Brachialaria', a
'Pluteus', is competent to produce the Echinoderm, which is so widely
different from it; if a hydroid polype can produce the higher Medusa;
if the vermiform Trematode 'nurse' can develop within itself the very
unlike 'Cercaria', it will not appear impossible that the egg, or
ciliated embryo, of a sponge, for once, under special conditions, might
become a hydroid polype, or the embryo of a Medusa, an Echinoderm."

It is obvious, from these extracts, that Professor Kolliker's hypothesis
is based upon the supposed existence of a close analogy between the
phenomena of Agamogenesis and the production of new species from
pre-existing ones. But is the analogy a real one? We think that it
is not, and, by the hypothesis, cannot be.

For what are the phenomena of Agamogenesis, stated generally? An
impregnated egg develops into an asexual form, A; this gives rise,
asexually, to a second form or forms, B, more or less different from
A. B may multiply asexually again; in the simpler cases, however, it
does not, but, acquiring sexual characters, produces impregnated eggs
from whence A, once more, arises.

No case of Agamogenesis is known in which, 'when A differs widely from
B', it is itself capable of sexual propagation. No case whatever is