Structural Variation among the Difflugian Rhizopods. 229 



primary chitinold 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 Diffiagia, 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 XVl. 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 llhizopodal 

 test (and of which the test is exclusively made up in certain 



