310 RANGE OF VARIATION, AND HABITS OF DIATOMACE.E. 



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 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. 142), 

 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 reduce the enormous number of 

 species with which our systematic 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 already 

 pointed out, being, probably, in a large proportion of cases, the 

 result of the multiplication of some one form, which, under modify- 

 ing 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 consider- 

 ing, since they are obviously more influenced than are those of 

 higher types by the conditions under which they are developed, 

 whilst, from the very wide Geographical range through which the 

 same forms are diffused, they are subject to very great diversities 

 of such conditions. 



235. The general habits of this most interesting group cannot 

 be better stated than in the w r ords of Prof. W. Smith. "The 

 Diatomaceae inhabit the Sea, or Fresh water; but the species 

 peculiar to the one are never found in a living state in any other 

 locality ; though there are some which prefer a medium of a mixed 

 nature, and are only to be met with in water more or less brackish. 

 The latter are often found in great abundance and variety in dis- 

 tricts occasionally subject to marine influences, such as marshes 

 in the neighbourhood of the sea, or the deltas of rivers, where, on 

 the occurrence of high tides, the freshness of the water is affected 

 by percolation from the adjoining stream, or more directly by the 

 occasional overflow of its banks. Other favourite habitats of the 

 Diatomacea? are stones of mountain streams or waterfalls, and the 

 shallow pools left by the retiring tide at the mouths of our larger 

 rivers. They are not, however, confined to the localities I have 

 mentioned, — they are, in fact, most ubiquitous, and there is hardly 

 a roadside ditch, water-trough, or cistern, which will not reward a 

 search, and furnish specimens of the tribe." Such is their abun- 

 dance in some Rivers and Estuaries, that their multiplication is 

 affirmed by Prof. Ehrenberg to have exercised an important influ- 

 ence in blocking-up harbours and diminishing the depth of channels ! 



