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 b, 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 roils 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 

 Difflugite should cease to be regarded as affbrding 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 Rhizopods, 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 

 Amoeba, the existence of any such determinate ratio becomes still 

 less admissible. It is therefore obvious that, in resorting to the 

 raeasui'ement 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 



