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 inorganic debris, 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 plahis 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 naturaUsts who are engaged in the study of the Rhizopods. 



15* 



