6l8 MICROSCOPIC FORMS OF VEGETABLE LIFE--THALLOPHYTES 



genus Mastogloia, which is specially distinguished by having the 

 annulus furnished with internal cost* projecting into the cavity of 

 the frustule, each frustule is separately supported on a gelatinous 

 cushion (fig. 465, B), which may itself be either borne on a branching 

 stipe (A), or may be aggregated with others into an indefinite mass 

 (fig. 466). The careful study of these composite forms is a matter 

 of great importance, since it enables vis to bring into comparison 

 with each other great numbers of frustules which have unquestionably 

 a common descent, and which must therefore be accounted 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 defi- 

 nition is altogether unsafe. Of the very strongly marked varieties 

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



PIG. 464. Schizonema Grevillii : A, natural size ; B, portion magnified five 

 diameters ; C, filament magnified 100 diameters ; D, single frustule. 



example in the valves C, D, E, F (fig. 465), which would scarcely 

 have been supposed to belong to the same specific type did they not 

 occur upon the same stipe. The careful study of these varieties in 

 every instance in which any disposition to variation shows itself, 

 so as to reduce the enormous number of species with which our sys- 

 tematic treatises are loaded, is a pursuit of far greater real value 

 than the multiplication of species by the detection of such minute 

 differences as may be presented by forms discovered in newly 

 explored localities; such differences as have already been 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 



