188 Transactions of the Boyal Microscopical Society. 
occupied the several positions, almost the universal condition of the 
field. Sometimes the zones were not complete, and sometimes 
there were two, or even one, instead of three ; but this largely 
depended upon the abundance, variety, and condition of the forms 
which might be taken up in any given dip." 
Now it would be an extremely difficult matter to prove that 
any arrangement approximate to this existed in a large macerating 
mass ; but we have nevertheless repeatedly observed that required 
forms were found in incomparably greater abundance in one part of 
the fluid than another. Be this, however, as it may, this zonal 
arrangement was a remarkable feature. What aggregated them 
together in this way it would be impossible to say satisfactorily ; 
probably the desire for oxygen may have drawn them towards the 
edge of the cover-glass ; but this will not account for their sifting 
out as it were into separate groups ; and it leaves unexplained how 
it was that creatures of higher organization, as ordinary para- 
moecia and forms of kerona, could live to the very end, in the 
greatest activity and apparent health, in the centre of the field, 
that is at A, Fig. 1, and its immediate neighbourhood. This at 
least we learned from it, that the earlier stages of many different 
forms were present, the adults of which may not at all he repre- 
sented in the field at the time. There are also, doubtless, myriads 
of other forms not recognizable, and we have given reason to 
believe * others still which to our best appliances are invisible. It 
is further in harmony with biological facts, and the evidence 
afforded by our inquiries, that many of these must await some 
change of circumstances before they could develop and come to 
maturity. The flagellate monads, for example, as a whole did not 
appear for some weeks after the various forms of bacteria ; and the 
calycine form did not in any case appear for some months after the 
" springing," hooked," and " biflagellate " monads. We do not 
attempt to explain this ; but we do not hesitate to believe that the 
reason is discoverable ; and it will in all probability be found to 
depend on very simple causes, such as the season and temperature, 
which is without question a real cause, the preparation of the 
pabulum by the bacteria and successive monads, and the chance 
addition of germs to the macerating mass, either through the air or 
otherwise. The two first reasons apply on the supposition that the 
fluid does contain the germs in a dormant condition until the 
circumstances become favourable. The third is a fact; for as we 
have already pointed out, the "biflagellate" monad was wholly 
absent one year, although its follower the " calycine " came in spite 
of its absence. In the same way we have observed the absence of 
the Spirillum volutans and other forms. It is true that this never 
* 'M. M. J.,' vol. ix., 1874, p. 71. 
