RABENHORST AND GRUNOW, ON DIATOMACEX. AZ 
both for the microscopist, who merely seeks for slides to 
gratify himself and his friends by the beauty and interest of 
their contents, and for the naturalist, who not only admires 
their structure, but endeavours to give a satisfactory reason 
for the various markings brought out by the higher powers 
of the microscope, to work out their habits and distribu- 
tion, and to reduce to some kind of systematic order, the 
various species which abound in different parts of the globe. 
With the Diatomacee, as with most other of the lowly 
organized forms of animal and vegetable life, 1t is a matter 
of extreme difficulty to arrive at any solution that will meet 
the approval of even the majority of observers, of the diffi- 
cult question—What is a species? Some, in fact, we may 
say many, regard every minute variation of form or marking 
as sufficient ground for the formation of a new species. 
Those, however, who have most carefully and extensively 
studied these organisms, by a critical comparison of large 
suites of specimens, by cbserving the variation arising from 
the localities in which they grow, and by studying the 
differences arising from the depth, temperature, and nature, 
of the water in which they are found, or from their living on 
sandy, muddy, or rocky shores, are led to the conclusion that 
a large number of these so-called species are mere varieties of 
some typical forms, whose growth has been stimulated or 
partially arrested by the favorable or unfavorable nature of 
the locality to the development of their siliceous skeleton. 
The majority of our English writers on this branch of na- 
_ tural history, since the time of Professor Smith, have been 
more engaged in the examination of gatherings, recent and 
fossil, and the descriptions of hitherto unrecognised forms, 
than in any attempt at systematising or working out the 
structure of the valves of the various species. The works at 
the head of this notice show, however, that in Germany the 
description of new species, though also largely indulged in, 
is combined with, and secondary to, more important objects. 
The local floras of Rabenhorst and Grunow giving very full 
and detailed description (in the latter case accompanied, 
generally, with very fairly executed figures) of the various 
species that occur in Saxony, Lusatia, Bohemia, and Austria, 
are very useful to students, for the sake of comparison 
with the species that occur in our own country, whilst, in 
the latest work of Dr. Rabenhorst, the ‘Flora Europea 
Algarum,’ the first section is devoted to a description of all 
the known European fresh and brackish water species of 
Diatomaceee, for which he proposes a new systematic ar- 
rangement, that differs materially from those of his prede- 
