129 
On the Development and Structure of the Diatom-valve * 
By G. C. Wallich, M.D., F.L.S. 
In a paper recently published in The Annals and Maga- 
zine of Natural History/ I endeavoured to show the unfit- 
ness of the valves of the Diatomacese as standard tests for 
microscopic lenses_, and based my objections to their 
employment on the variable character of the markings in 
different individuals of the same species^ in the same species 
under varying conditions of development, and in the same 
specimen under diflPerent methods of examination. In deal- 
ing with this subject, I purposely selected Pleurosigma angu- 
latum and P. balticum, the forms most frequently referred to, 
without assuming, for a moment, that the markings of these, 
or any other diatoms, are of sufficient importance, in a bio- 
logical point of view, to entitle them to interest ; and solely 
because it became imperative on me, whilst condemning tests 
hitherto relied on, to prove that some of those most constantly 
in use, and in the delineation and description of which the 
greatest pains has been taken by writers, exhibit characters 
irreconcilable with the structure usually assigned to them. 
In the following observations, I shall principally endeavour 
to prove that growth does not take place in the diatom valve, 
after its primary development ; and that the variation observ- 
able in the size and markings of different individuals of the 
same species is not only consistent with this view, but 
naturally follows from it. 
The mere discussion as to whether the surface of a valve 
exhibits this or that kind of markings, so long as no higher 
purpose is in view than the production of certain appearances 
under the microscope, and the claim of superiority for the 
instrument whereby the most Protean aspects are engendered, 
must always remain barren of scientific results. For we are 
bound not only to see, but to comprehend the relation of one 
portion of minute structure to another, before we are in a 
position to draw serviceable inferences from it. It is well 
understood, by all who have had experience in microscopic 
manipulation, that great caution is necessary in pronouncing 
upon the precise structure of an object ; and that, under 
imperfect adjustments, either as regards the object or the 
instrument, an indefinite amount of variation may be made 
apparent. This is especially the case under the employment 
of the higher powers. For, although the definition of the 
entire series of objectives, manufactured by our leading 
opticians, may be said to be equally perfect, a variety of 
