Structural Variation among the Difflugian Rhizopods. 237 



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 Diffiugia and 

 other testaceous forms throws no light on this subject; for there 

 are no means of knowing whether such tests have been shed, 

 accoi-ding 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 Difflugida 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 even in 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 Alga? 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 1 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 unlocked 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 



* In 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. 



