236 Dr. G.C. Wallich on the Extent and Causes of 
single variety or series of varieties, but pervade the whole group, 
inasmuch as I have met with tests of the mitriform series con- 
taining the encysted animal, the posterior third of which was still 
covered with large unmetamorphosed sandy particles, whilst the 
anterior two-thirds were studded with the elongated discs of 
figs. 28&29. In these tests the apertures were in every respect 
similar to the one figured at fig. 30 4, thus proving the transition 
from the common pyriform Difflugian test to that variety in 
which all appreciable trace of mineral matter is lost. The above 
facts also enable us to account for the very common and pro- 
voking occurrence with which most persons who are acquainted 
with the freshwater testaceous Rhizopods must be familiar, 
namely, the frequent impossibility of getting the animal to 
emerge from the ball into which it rolls itself in the interior of 
the test. 
These partially and wholly closed orifices are instructive, how- 
ever, from another point of view, namely, from their proving 
that, even in the most mature state of the Difflugian test, its out- 
line is liable to change, and hence that the external mechanical 
agencies to which I have adverted in the case of D. spiralis may 
actually produce the duplicature of its test. 
We now come to modifications in size. Were more cogent 
reasons wanting why mere measurements of the tests of the 
Difflugie should cease to be regarded as affording a basis for 
specific distinction, the facts already advanced to show how 
largely the whole of the external characters of the tests are 
influenced by outward conditions, which are themselves of the 
most fluctuating nature, would, as I conceive, amply suffice for 
the purpose. It is well known, however, that amongst the 
freshwater testaceous Khizopods, the actual bodies of the 
animals occupy but a small and indeterminate portion of the 
chamber in which they are encased; and I presume no one will 
maintain the existence of a fixed ratio between the animal and 
its test. Should fission occur in Difflugia, as we know it does in 
Ameba, the existence of any such determinate ratio becomes still 
less admissible. It is therefore obvious that, in resorting to the 
measurement of the test as a criterion of the dimensions of the 
animal, the process must be altogether fallacious. But even 
amongst the higher tribes of the animal and vegetable king- 
doms, notwithstanding the facilities these offer for ascertaining 
when the extreme limits are arrived at either in growth or age, 
size is not accepted as a test of specific unity or distinctness, 
‘ 
unless it be accompanied by such structural or functional — 
changes as can be shown to have exerted an influence in modi- 
fying it. But amongst the microscopic forms of life we have 
hitherto entirely failed to trace any constant indication by means 
