Structural Variation among the Difflugian Rhizopods. 287 
of which it is possible to determine whether a specimen under 
examination be a mature or an immature one. The frequent 
occurrence in the muddy deposits of effete tests of Difflugia and 
other testaceous forms throws no light on this subject ; for there 
are no means of knowing whether such tests have been shed, 
according to a periodically recurring influence, or are merely 
left after the death of the occupant at a certain stage of its 
existence or through accident ; and until such an indication is 
forthcoming, all specific distinctions based on mere size must 
therefore be valueless. These remarks apply, however, only to 
the measurements of such objects as the tests of the Difflugide or 
other Rhizopods, and not to special organs, which, as they 
rarely vary to any great extent, whether in young or old indivi- 
duals, may hence be frequently recognized by their dimensions 
alone. But evenin such cases the aid afforded by measurement 
must continue to be counterbalanced so long as no uniform 
scale is adopted both in descriptions and figures, and a certain 
amount of calculation is necessary before we can arrive at the 
fraction which expresses what we desire to ascertain. 
Of one fact I have had abundant opportunity of satisfying 
myself, namely, that the dimensions of the Difflugian tests, in 
like manner with their plan of growth and external characters, 
are modified to an extraordinary degree by the nature of the 
localities in which they happen to be found,—still water, with an 
abundance of food in the shape of minute Alge and Infusoria, 
constituting the most favourable conditions; whilst variations 
in climate would seem to influence their growth and increase 
only in an indirect manner, namely, by increasing or diminish- 
ing the quantities of sustenance. 
As stated in an early portion of these observations, I have 
met with representatives of every variety of the freshwater 
testaceous Rhizopods in each of the remote regions of the globe 
in which I have searched for them. It is well known that 
Diatoms, a group of organisms holding a position in the vege- 
table kingdom probably parallel to that held by the Rhizopods 
in the animal, are to be found in all climates. It is an interest- 
ing fact, however, and one which was somewhat unlooked for, 
that, under the conditions prevailing in high northern latitudes, 
the long-continued congelation to which the whole of the lower 
forms of life are annually subject seems to exercise no destruc- 
tive effect ; for not only are the freshwater Diatoms and Desmi- 
dians* very plentiful, but also the whole of the freshwater 
* Tn one locality in West Greenland, at an elevation of probably about 
1000 feet above the sea, I obtained no less than twenty-six species and va- 
rieties of Desmidians. Had my object been to collect this kind of organ- 
ism, the number might doubtless have been largely augmented. 
