MICROSCOPIC FORMS OF VEGETABLE LIFE. 301 



obviously more influenced than 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. 



298. The general habits of this most interesting group cannot be 

 better stated than in the words of Prof. W. Smith. "The Diatomaceaa 

 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 districts occasionally subject to marine influ- 

 ences, 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 

 Diatomaceae 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 abundance in some rivers and estuaries, 

 that their multiplication is affirmed by Prof. Ehrenberg to have exercised 

 an important influence in blocking up harbors and diminishing the 

 depth of channels! Of their extraordinary abundance in certain parts 

 of the Ocean, the best evidence is afforded by the observations of Sir J. 

 D. Hooker upon the Diatomaceae of the southern seas; for within the 

 Antarctic Circle they are rendered peculiarly conspicuous by becoming 

 inclosed in the newly formed ice, and by being washed up in myriads by 

 the sea on to the 'pack' and 'bergs/ everywhere staining the white ice 

 and snow of a pale ochreous brown. A deposit of mud, chiefly consisting 

 of the siliceous loricae of Diatomaceae, not less than 400 miles long and 

 120 miles broad, was found at a depth of between 200 and 400 feet, on 

 the flanks of Victoria Land in 70 South latitude. Of the thickness of 

 this deposit no conjecture could be formed; but that it must be contin- 

 ually increasing is evident, the silex of which it is in a great measure 

 composed being indestructible. A fact of peculiar interest in connection 

 with this deposit, is its extension over the submarine flanks of Mount 

 Erebus, an active Volcano of 12,400 feet elevation; since a communica- 

 tion between the ocean waters and the bowels of a volcano, such as there 

 are other reasons for believing to be occasionally formed, would account 

 for the presence of Diatomaceae in volcanic ashes and pumice, which was 

 discovered by Prof. Ehrenberg. It is remarked by Sir J. D. Hooker, 

 that the universal presence of this invisible vegetation throughout the 

 South Polar Ocean is a most important feature, since there is a marked 

 deficiency in this region of higher forms of vegetation; and were it not 

 for them, there would neither be food for aquatic Animals, nor (if it 

 were possible for these to maintain themselves by preying on one another) 

 could the Ocean waters be purified of the carbonic acid which animal 

 respiration and decomposition would be continually imparting to them. 

 It is interesting to observe that some species of marine Diatoms are 

 found through every degree of latitude between Spitzbergen and Vic- 

 toria Land, whilst others seem limited to particular regions. One of the 

 most singular instances of the preservation of Diatomaceous *forms, is 

 their existence in Guano; into which they must have passed from the 



