130 
Wallich, on the Diatom-valve. 
conditions are essential to secure accuracy of observation^ and 
these can only be fulfilled under the exercise of the strictest 
vigilance and judgment. 
But whilst the minute structure of the diatom-valve may 
be deemed by some persons unworthy of the labour that has 
been bestowed upon it, it is indisputable that the correct ex- 
position of that structure involves a question, quite as impor- 
tant, perhaps, as any we have to encounter in the whole course 
of vegetable physiology ; and however cynically some of our 
intellectual eagles may affect to treat the subject, they may 
rest assured that no little honour awaits the observer who 
shall place the laws by which it is governed in an intelligible 
light. 
Why, let me ask, does one organism present peculiarities 
in the arrangement of its cell-wall which do not occur in 
another immediately allied to it ? Why do we find organisms 
belonging to the same group, formed on the same general 
plan, surrounded by the same influences, and deriving 
nourishment from the same medium, vary so remarkably in 
the disposition of one of their elements, in which we should 
least expect to meet with variability? Thus, amongst the 
Diatomacese, why do we observe such differences between the 
arrangement of the siliceous particles in Triceratmm and 
Campylodiscus , Campylodiscus and Pleurosigma, or Pleurosigma 
and Pinnularia, and so on with the remaining genera ? As 
nothing in nature is, or indeed can be, fortuitous, some law 
must operate in bringing about these differences. Although 
as yet ignorant of the nature of that law, there is no reason 
whatever why we should remain so. Day after day brings 
forth fresh facts, in elucidation of the complicated scheme of 
creation ; and no one can tell, therefore, how great a length 
of the chain of knowledge may be consolidated, by the addi- 
tion of a link snatched from objects holding no higher place 
in the scale than the unpretending diatom. 
Other more startling discoveries have been effected, and 
have earned for the discoverers greater eclat ; but it is 
doubtful whether any have surpassed in physiological value 
Schwann and Schleiden^s enunciation of the theory of cell- 
formation. On this basis the study of the entire organic 
world may be said to rest. It is the groundwork of our 
acquaintance with the structure of every living thing, from 
the lowest protophyte up to man. But there is a link, yet 
beyond that point, which requires to be filled up and riveted. 
I allude to the law that presides over the development of the 
cell-wall ; and this, I conceive, can hardly be studied more 
advantageously than amongst the Diatomaceae, the indestruct- 
