GENERAL DISTRIBUTION OF DIATOMACBiB. 301 



tion to variation shows itself, so as to reduce the enormous num- 

 ber 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 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 previ- 

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

 such humble organisms as those we have been considering ; 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. 



190. The general habits of this most interesting group cannot 

 be better stated than in the words of Prof. W. Smith, from 

 whose admirable Monograph the Author has drawn the greater 

 part of his materials for the foregoing account of it.^ " The 

 Diatomacese inhabit the sea, or fresh water ; but the species pe- 

 culiar 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 districts occasionally subject to marine influences, such 

 as marshes in the neighborhood 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 

 favorite habitats of the Diatomacese 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, how- 

 ever, confined tp 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 abundance in 

 some rivers and estuaries, that their multiplication is afiirmed 

 by Prof. Ehrenberg to have exercised an important influence in 

 blocking up harbors and diminishing the depth of channels ! 



' The Author has great pleasure in here acknowledging the liberality of Messrs. 

 Smith and Beck, who have allowed him, not only to make free use of this volume, but 

 also to copy as many as he desired of the admirable series of illustrations which have 

 been executed for it by Mr. Tuffen West, many of them still unpublished. All the 

 figures of Diatomacese, given in' this Manual, except Figs. 80 and 85, which are drawn 

 from nature, and Figs. 101, 102, which are copied from Prof. Ehrenberg, are as exact 

 copies of Mr. West's lithographs as the best Wood-engraving can produce. 



