Structural Variation among the Difflugian Rhizopods. 219 
modifications in colour, arising partly from the nature of the food 
that has been incepted by the animal, partly from the external 
incrustation of organic or imorganic débris, and occasionally 
from the depth of tint of the chitinoid basal substance. 
Before proceeding, however, I may be permitted to state that 
the whole of the forms about to be described in these pages 
have not only been met with by me abroad, in the plains of 
Lower Bengal, in the Himalayan Mountains, on the confines 
of the Arctic Circle, in Labrador, and in Nova Scotia, but, 
excepting the two or three extreme varietal forms encoun- 
tered only in the most remote of these localities, the whole 
are to be found abundantly in nearly every pool and streamlet 
throughout England. It may also be mentioned that my con- 
clusions have in no case been arrived at from the examination of 
solitary specimens, but only after a laborious comparison of 
those which have occurred in sufficient numbers and in sufficiently 
varying stages of growth to guarantee the avoidance of excep- 
tional examples *. 
Taking the several kinds of varietal modification referred to 
in the order of their importance, I have, in the first place, to 
notice the one involving the shape of the test, or, as Dr. Car- 
penter appropriately designates it, in speaking of the Foramini- 
fera, “their plan of growth.” For if, as he has urged, this fur- 
nishes no natural basis for generic distinction in the family to 
which he more especially refers, in which the structure of the 
shell is at times highly complex, it is obvious that the plan of 
growth cannot afford a trustworthy distinctive character in a 
group of organisms the tests of which are of the simplest kind, 
and hence more liable to come under the operation of varying 
physical influences. 
After having most carefully studied the several varieties of 
Difflugian test, and compared the characters of the whole series 
with due regard to the local conditions under which they were 
found, I have been irresistibly led to the conclusion that they 
have sprung from one primary or larval form ; in other words, I 
conceive that the whole are referable to a single specific type, 
and that, whatever varietal figure the mature test assumes, such 
figure may be the result of peculiarities in the external con- 
ditions by which it is surrounded, and not the result of heredi- 
_ tary transmission in each case. So that, supposing A to repre- 
sent the prevailing form of the Difflugian test in its earliest 
stage, under the varying conditions of the medium by which it 
* The foreign materials to which I have just alluded still remain in my 
possession, in a mounted as well as an unmounted state; whilst all the 
specimens from which I have made my drawings are preserved for verifi- 
cation by those naturalists who are engaged in the study of the Rhizopods. 
* 
