218 Dr. G.C. Wallich on the Extent and Causes of 
tapering to some extent, radiate, and freely coalescent ; while, 
coupled with and perhaps dependent on these characters, the 
pseudopodia of this subfamily, as if to compensate for the re- 
stricted power of locomotion possessed by them in comparison 
with the Amebe proper, are much more active—the rapidity 
with which they admit of beg projected outwards or withdrawn 
into the test being unequalled in any other form, and presenting 
the most wonderful example of inherent contractility in an 
amorphous animal substance that is to be met with in either 
of the great organic kingdoms. It is hardly necessary to point 
out, however, that, in the absence of other characters, this 
peculiarity can no more be regarded as indicative of generic, or 
even specific, distinctness than the difference in the speed of the 
racer and cart-horse can be regarded as indicating their specific 
separation. 
But it is in the features presented by the tests of the Difflu- 
gian group of freshwater Rhizopods that the tendency takes 
place to the inordinate modification to which allusion has been 
made, and enables us to perceive that it is dependent on the 
fluctuating condition of the water in which they live and of the 
mineral ingredients peculiar to the soil of each locality. For, as 
might naturally be expected from the protection afforded by the 
tests, the soft parts are uninfluenced by many of those agencies 
which produce changes in the external characters of the naked 
genera; and hence we are enabled the more readily to assure 
ourselves of the unity, in each case, of the generic type which 
pervades the two groups into which these organisms appear to 
me naturally to resolve themselves. 
I propose, in the first place, to adduce the grounds on which I 
base the opinion that, with the exception of a few permanent varie- 
ties which exhibit a type capable of being hereditarily transmitted, 
the whole of the varieties may be regarded as the result, 1st, of 
modifications in figure, dependent in some instances on the inabi- 
lity of the test to sustain its own weight, and in others on its 
tendency to assume curvature or obliquity from the action of 
running water ; 2ndly, of modifications in the materials of which 
the tests are constructed, depending sometimes on the kind of 
mineral substances procurable in particular localities, sometimes 
on a hitherto unrecognized and very remarkable union between 
the chitinoid basal substance (which is an exudation from the ani- 
mal) and the mineral particles which that substance serves in the 
first instance merely to cement together ; 3rdly, of modifications 
in size, depending probably on the age, the perfect or imperfect 
nutrition of the individual, and also on the capability of the test 
to alter its form after having become consolidated to a certain 
extent by the addition of mineral particles ; 4thly and lastly, of 
