But let me give a practical instance of how what appears like fact may
override philosophy, if an incident, or even a group of incidents,
_per se_ are to control our judgment.

Eighteen years ago I was paying much attention to vorticellæ. I was
observing with some pertinacity _Vorticella convallaria_; for one of
the calices in a group under observation was in a strange and
semi-encysted state, while the remainder were in full normal activity.

I watched with great interest and care, and have in my folio still the
drawings made at the time. The stalk carrying this individual calyx
fell upon the branch of vegetable matter to which the vorticellan was
attached, and the calyx became perfectly globular; and at length there
emerged from it a small form with which, in this condition, I was
quite unfamiliar; it was small, tortoise-like in form, and crept over
the branch on setæ or hair-like pedicels; but, carefully followed, I
found it soon swam, and at length got the long neck-like appendage of
_Amphileptus anser_!

Here then was the cup or calyx of a definite vorticellan form changing
into (?) an absolutely different infusorian, viz., _Amphileptus
anser_!

Now I simply reported the _fact_ to the Liverpool Microscopical
Society, with no attempt at inference; but two years after I was able
to explain the mystery, for, finding in the same pond both _V.
convallaria_ and _A. anser_, I carefully watched their movements, and
saw the _Amphileptus_ seize and struggle with a calyx of
_convallaria_, and absolutely become encysted upon it, with the