developed teeth, even teeth of the second dentition, in ovarian
tumours,[897] are facts leading to the same conclusion.

Whether each of the innumerable autonomous elements of the body is a cell
or the modified product of a cell, is a more doubtful question, even if so
wide a definition be given to the term, as to include cell-like bodies
without walls and without nuclei.[898] Professor Lionel Beale uses the term
"germinal matter" for the contents of cells, taken in this wide
acceptation, and he draws a broad distinction between germinal matter and
"formed material" or the various products of cells.[899] But the doctrine
of _omnis cellula e cellula_ is admitted for plants, and is a widely
prevalent belief with respect to animals.[900] Thus Virchow, the great
supporter of the cellular theory, whilst allowing that difficulties exist,
maintains that every atom of tissue is derived from cells, and these from
pre-existing cells, and these primarily from the egg, which he regards as a
great cell. That cells, still retaining the same nature, increase by
self-division or proliferation, is admitted by almost every one. But when
an organism undergoes a great change of structure during development, the
cells, which at each stage are supposed to be directly derived from
previously-existing cells, must likewise be greatly changed in nature; this
change is apparently attributed by the supporters of the cellular doctrine
to some inherent power which the cells possess, and not to any external
agency.

Another school maintains that cells and tissues of all kinds may be formed,
independently of pre-existing cells, from plastic lymph or blastema; and
this it is thought is well exhibited in the repair of wounds. As I have not
especially attended to histology, it would be presumptuous in me to express
an opinion on the two opposed doctrines. But every one appears to admit
that the body consists of a multitude of "organic units,"[901] {371} each
of which possesses its own proper attributes, and is to a certain extent
independent of all others. Hence it will be convenient to use indifferently
the terms cells or organic units or simply units.

_Variability and Inheritance._--We have seen in the twenty-second chapter
that variability is not a principle co-ordinate with life or reproduction,
but results from special causes, generally from changed conditions acting
during successive generations. Part of the fluctuating variability thus
induced