382 Dr. H. Charlton Bastian on the Origin 
But this kind of proof is impossible in the case of Bacteria. 
Where the proof is most direct (even where the birth of 
Bacteria seems to take place under our eyes) it must always 
be a question of particles of living matter emerging from the 
region of the invisible—appearing, that is, where previously 
visible particles were absent. While in other cases, without 
being so directly present as it were at their birth, we may 
find them existing in parts of animals or plants, after these 
have been subjected to certain experimental conditions, where 
previously, in accordance with all existing knowledge and 
belief, neither they nor their germs are to be found. They 
may appear, that is, when such animals or plants, or, rather, 
portions of them, have been exposed to certain experimental 
conditions unfavourable to their pre-existing vital processes, 
but which yet, as we assume, allow the constituent proto- 
plasm to die more or less slowly, and portions of it to indi- 
vidualize themselves and grow into this or that form of 
microorganism. 
The assumption here is that we have, as starting-points, to 
do with pre-existing units of living matter. Still, in the 
cases where our observations are made upon protoplasm 
which appears to be actually structureless, or upon one or 
other of the fluids pertaining to an animal or plant, the actual 
mode of origin, so far as appearances go, may be precisely 
similar to that which would occur in archebiosis ; in each 
case there would be the appearance of particles where none 
previously existed. The difference between the two cases 
would be this: in archebiosis we should have to do with the 
actual birth of units of living matter, with a synthesis, that 
is, from its elements; but in the case of heterogenesis we 
have confessedly to do with living matter already existing, so 
that we postulate only an individualization of living particles 
or of larger units, together with a change in their mode of 
life. 
Where the particles of living matter so individualizing 
themselves are so small as to be beyond the range of our 
most powerful microscopes, it would be impossible to say, as 
they grow and become recognizable im organic fluids, whether 
newly appearing particles had arisen by heterogenesis or by 
archebiosis. On the other hand, it must be recognized that 
there are particles of various kinds in the tissue elements and 
fluids of an animal or plant which it is often impossible by 
mere microscopical examination to discriminate from germs 
of microorganisms. And until such germs begin to grow 
and assume specific shapes, or particular collocations, their 
discrimination from particles which are normal constituents 
of such tissues or fluids cannot safely be made. 
