on the Extent and Causes of 
appeared in the ‘ Annals’ during the course of the bygone year. 
On the present oceasion I propose to show that amongst the 
testaceous genera of the same order (namely, the Proteina*) this 
tendency to variation is no less marked, and that it is attribut- 
able (as was stated to be the case in the non-testaceous genera) 
rather to the ever-fluctuating conditions of the medium by which 
the organisms are surrounded than to any special hereditary 
idiosynerasies by which certain characters become impressed upon 
certain races of individuals. 
The primary points to be considered are, whether the degree 
of variation observable in the soft parts—or actual bodies of these 
creatures—is at all commensurate with that which is traceable in 
their tests; and whether there exists such an intimate relation 
between the changes in figure, composition, size, and colour of 
the tests, and of the sarcode-bodies over which they are formed, 
as to indicate varying degrees of physiological advance or de- 
gradation. For it cannot be too strongly urged that, on the 
proper apprehension of these two questions must mainly depend 
our ability to discriminate between characters that are, and those 
that are not, of true specific value; and that every incautious 
addition to our lists of species, whilst inevitably operating as 
an obstacle in the path of those persons who are desirous of 
studying the biological relations of the lower forms of organic 
being, must also tend materially to augment the difficulties which 
stand in the way of a natural and easily available systematic 
arrangement. 
As stated by me in the ‘Annals’ for June last (p. 452), 
it seems almost impossible to examine the sarcode-body of 
Difflugia and Arcella, on the one hand, and of Huglypha and 
some allied forms, on the other, without perceiving that in 
each case these organisms ought to be comprehended under a 
single genus. Between the degree of differentiation attained by 
the sarcode of Difflugia and Arcella there is nothing suggestive 
of more than specific distinctness. Both these forms are Amebe 
in the strictest sense; that is to say, their bodies consist of sar- 
code in which the degree of consolidation of the external layer 
for the time being is so complete that the outline of the surface, 
* Under this designation are united by me those Rhizopods which differ 
from the preceding orders (namely, the Herpnemata and Protodermata) in 
the possession both of a definite nucleus and contractile vesicle. They are 
divisible into two primary families, the ‘“Amcebina”’ and “ Actinophryna,” 
based principally on peculiarities presented by the sarcode-body. 
The Diffugide constitute a subfamily of the Amebina standing natu- 
rally apart from the remaining testaceous forms which present the Actino- 
phryan type of ectosare and pseudopodia, and hence coustitute a subfamily 
of the Actinophryna under the name of the Euglyphide. (See classifica- 
tion of these subfamilies, at page 240). z 
