Structural Variation among the Difflugian Rhizopods. 229 
primary chitinoid wall of the chamber, a series of irregular 
micaceous-looking plates. Indeed it is difficult to conceive 
how additions could be made externally to the thickness of the 
test by any other method than the one indicated,—the accumu- 
lation of layer upon layer of mineral particles being, in some 
cases, carried to such an extent that the wall of the test attains 
considerable thickness and an outline so bold and rugged as to 
present the appearance, under the microscope, of a mass of 
coarse sandstone rather than a built-up chamber tenanted by 
a living organism. 
I have never succeeded in actually witnessing the process of 
test-construction going on in Difflugia. But were the particles 
of mineral matter conveyed to their resting-places by the pseudo- 
podia, it seems hardly possible that it should have escaped 
notice altogether ; and, on this account, I am rather inclined to 
think that, whilst the creature is enabled by means of these 
organs to select such particles as are best fitted for its purpose, 
they serve merely to drag the body towards the particles, and 
eventually to bring that portion of the test upon which they are 
destined to be lodged in contact with them. 
No doubt this presupposes a selective power far in advance of 
any faculty we should a priori be inclined to attribute to organ- 
isms of so rudimentary a type. But to deny this power is simply 
to deny an established fact which can be accounted for in no 
other way. We have only to examine such tests as are represented 
in Plate XVI. figs. 9, 10, 16, 20, 22, &c., to satisfy ourselves that 
the collection of mineral particles of certain dimensions, and of cer- 
tain kinds, must necessarily have been effected with a view to serve 
some particular purpose. But, wonderful in itself as this faculty 
must appear, I beg to draw special attention to it, inasmuch as it 
tends, in conjunction with other distinctions to which it is unne- 
cessary for me at present to refer, to establish the impassable 
boundary-line betwixt the animal and the plant—between the 
manifestation of vital, chemical, and physical agencies on the one 
hand, and these combined with psychical agencies on the other. 
And I venture to say that, however stubbornly we may ignore this 
doctrine, simply on the score that it has heretofore defied our 
comprehension, the day will assuredly come when, with the as- 
sistance of a more perfect knowledge of organic life than we 
as yet possess, its accuracy shall cease to be impugned. 
The next series of varietal modifications, in the order of their 
importance, is that involving the materials of which the Difflugian 
test is constructed. 
I have, thoughout these observations, spoken of the exudation 
from the animal which constitutes the basis of every Rhizopodal 
test (and of which the test is exclusively made up in certain 
