318 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 Amoeba proper, are much fliore active — the rapidity 

 with which they admit of being projected outwards or withdrawn 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 



