318 Dr. Wallicli on the RMzopods. 



in any particular group of organisms the greater being the 

 evil etFect of ignoring these considerations. Now it is univer- 

 sally allowed by all who have systematically studied the tes- 

 taceous Amcebans, that these organisms are, of all others, the 

 most liable to extreme variation, in virtue of their being the 

 most likely to be affected by external conditions and purely 

 local influences. 



Ehrenberg, the great pioneer in microscopic natural history, 

 in touching upon this subject observed that " in the remark- 

 able mode of reproduction by self-division and the indiffe- 

 rence of these minute independent beings to climatic variation 

 there appear to reside characters which sufficiently distinguish 

 them from larger beings, so as to make them preeminently 

 adapted to a greater duration and extension through entire 

 and successive formation-epochs of the earth." — PhiL Mag. 

 (from Trans. Roy. Acad. Berlin, 1840). 



This tersely-expressed opinion has been repeatedly borrowed 

 by later writers without due acknowledgment, and coupled 

 with occasional additions and alterations, which have not 

 tended to improve, but to impair, its import. In allowing 

 myself to render it more closely applicable to the particular 

 group of organisms forming the subject of the present 

 inquiry, it is my earnest wish not to fall under any such 

 imputation. 



The causes affecting the stability, extension by variation^ 

 and extinction of the Protozoan species follow a law which may 

 be thus stated : — The lower the type the less liable is it to be- 

 come extinct, but the more liable is it to undergo what may be 

 termed constructive variation, inasmuch as its simple body- 

 substance is least powerfully affected by changes in the 

 material condition of the medium in which it lives, whereas 

 its protective covering (should it possess one), the basis of 

 which is invariably chitinoid, and consists of a permanently 

 consolidated layer of ectosarc thrown off from the animal 

 itself, is the first portion to be acted on by extraneous condi- 

 tions. We are thus enabled to explain why the body- 

 substance of the testaceous Ehizopods remains unaltered, 

 whereas their protective covering presents an almost infinite 

 varietal range both as regards the materials of which it 

 is constructed and the form the construction assumes. 



With these preliminary remarks before us, let us now inquire 

 how far the most commonly accepted subdivision of the Khi- 

 zopods into orders, viz. that proposed by Dr. W. B. Carpenter, 

 can be considered a natural one, bearing in recollection, 

 however, that it is to the generic and specific subdivisions of 

 the two most thoroughly known families, namely tliose 



