on and fission; for in the latter case the divided portion, however
small, is more perfectly organised; but most physiologists are now
convinced that the two processes are essentially alike.[875] Prof. Huxley
remarks, "fission is little more than a peculiar {359} mode of budding,"
and Prof. H. J. Clark, who has especially attended to this subject, shows
in detail that there is sometimes "a compromise between self-division and
budding." When a limb is amputated, or when the whole body is bisected, the
cut extremities are said to bud forth; and as the papilla, which is first
formed, consists of undeveloped cellular tissue like that forming an
ordinary bud, the expression is apparently correct. We see the connection
of the two processes in another way; for Trembley observed that with the
hydra the reproduction of the head after amputation was checked as soon as
the animal began to bud.[876]

Between the production, by fissiparous generation, of two or more complete
individuals, and the repair of even a very slight injury, we have, as
remarked in a former chapter, so perfect and insensible a gradation, that
it is impossible to doubt that they are connected processes. Between the
power which repairs a trifling injury in any part, and the power which
previously "was occupied in its maintenance by the continued mutation of
its particles," there cannot be any great difference; and we may follow Mr.
Paget in believing them to be the selfsame power. As at each stage of
growth an amputated part is replaced by one in the same state of
development, we must likewise follow Mr. Paget in admitting "that the
powers of development from the embryo are identical with those exercised
for the restoration from injuries: in other words, that the powers are the
same by which perfection is first achieved, and by which, when lost, it is
recovered."[877] Finally, we may conclude that the several forms of
gemmation, and of fissiparous generation, the repair of injuries, the
maintenance of each part in its proper state, and the growth or progressive
development of the whole structure of the embryo, are all essentially the
results of one and the same great power.

_Sexual Generation._--The union of the two sexual elements seems to make a
broad distinction between sexual and asexual reproduction. But the
well-ascertained cases of Parthenogenesis prove that the distinction is not
really so great as it at first appears; for ovules occasionally, and even