The Development, &c., of Diatomacee. By Dr.G. C. Wallich. 81 
power has been expended), there cannot be a doubt that the new 
brood may vary very considerably in its dimensions. And it is 
here, and here only, as I clearly pointed out (though Dr. Mac- 
donald no doubt quite unwittingly misunderstood my words), that 
“vicissitudes of season” come into operation. It is, I believe, true, 
that mere climate has little if anything to do with the matter. 
IT have found almost as exuberant growths of Desmidiacex in Green- 
land and Labrador, as in the Tropics. _ It is the extreme and sudden 
vicissitudes of heat and cold, of drought and deluging rain, that 
are the chief modifying agents which tell upon the expectant 
Sporangia. And even in the sub-arctic lands referred to, such 
vicissitudes occur, for during a very few hours of the brief summer 
day the heat is sometimes actually excessive. 
Taking all these circumstances into due consideration, I venture 
to submit that from the birth of the diatom to its death, influences 
are at work which are, at all events, more than sufficient to render 
that mathematical uniformity in the markings of the diatom valve 
impossible, which is an essential condition if they are to be used as 
test-objects. And I would therefore sum up my observations on 
this portion of my subject by repeating one of the opening para- 
graphs of my paper “On the Markings of the Diatomacex in 
common use as Test-objects,” published in ‘The Annals and Mag. 
Nat. Hist. for February 1860: 
“Certain diatoms may still be advantageously employed as test- 
objects, but assuredly not in the manner hitherto in vogue. In 
order to ensure uniformity, or, what amounts to the same thing, 
to ensure that the purchaser of a lens of a stated power shall 
actually obtain what he desires, it becomes essential that each test- 
slide should itself be first compared with some accredited and 
universal standard, before being applied for determining the capa- 
bilities of any optical combination.” 
