300 



THE MICROSCOPE AND ITS REVELATIONS. 



ported on a gelatinous cushion (Fig. 189, B), which may itself be either 

 borne on a branching stipes (A), or may be aggregate! with others into 

 an indefinite mass (Fig. 190). The careful study of these composite 

 forms is a matter of great importance; since it enables us to bring into 

 comparison with each other great numbers of frustules which have 

 unquestionably a common descent, and which must therefore be ac- 

 counted as of the same species; and thus to obtain an idea of the range 

 of variation prevailing in this group, without a knowledge of which 

 specific definition is altogether unsafe. Of the very strongly marked 

 varieties which may occur within the limits of a single species, we have 

 an example in the valves c, D, E, F (Fig. 189), which would scarcely have 

 been supposed to belong to the same specific type, did they not occur 

 upon the same stipes. The careful study of these varieties in every 

 instance in which any disposition to variation shows itself, so as to 



FIG. 189. 



. 190. 



Fig. 189. Mastogloia Smithii .A, entire stipes ; B, f rustule in the gelatinous envelope ; c F, dif- 

 ferent forms of f rustule as seen in side view; G, front view; H, f rustule undergoing subdivision. 

 Fig. 190. Mastogloia lanceolata. 



reduce the enormous number of species with which our systematic 

 treatises are loaded, is a pursuit of far greater real value than the multi- 

 plication of species by the detection of such minute differences as may be 

 presented by forms discovered in newly explored localities; such differ- 

 ences, as already pointed out, being, probably, in a large proportion of 

 cases, the result of the multiplication of some one form, which, under 

 modifying influences that we do not yet understand, has departed from 

 the ordinary type. The more faithfully and comprehensively this study 

 is carried out in any department of Natural History, the more does it 

 prove that the range of variation is far more extensive than had been 

 previously imagined; and this is especially likely to be the case with such 

 humble organisms as those we have been considering, since they are 



