Wallich, on the Diatom-valve. 
135 
variation in dimensions that occurs so frequently ? If not 
to growth^ in the ordinary sense^ to what other cause ? It is 
attributable, in the first place, to increased or diminished 
supply of nutrition to which the species happens to be subject; 
acting by the production of differences between the parent 
and the young valve, wholly inappreciable to our vision, not- 
withstanding all our appearances, but yet quite capable of 
effecting the variation in question, through the intervention 
of a multitude of individuals. In strict truth, no two valves 
of the same frustule can be of the same size ; for the new 
valves, being formed within the connecting zones of the parent 
frustule, must be smaller than these. In answer to this it 
may be urged that, in the usual course of growth, they reach 
the dimensions of the parent valve. But unless we are pre- 
pared to admit that the latter obligingly cease growing for a 
time, to permit of the requisite uniformity in size being 
attained, it will be seen that this objection is invalid. The 
difference in the two valves arising from the last-mentioned 
cause, however infinitessimal it may appear in the case of the 
individual, becomes, nevertheless, all-powerful when operating 
through vast successions of individuals ; and is, therefore, of 
itself sufficient to account for the variations we witness. 
The main source of difference, however, in the size of the 
valves of any given species is derived from the peculiar 
idiosyncrasy of the sporangial frustule. The large dimensions 
that frustule attains in many cases is well known. And, 
although the precise history of the produce of the sporangial 
cell still remains doubtful, there is, I believe, quite sufficient 
evidence forthcoming to show that the prevailing opinion, as 
to the great variation in dimensions of the new brood, is quite 
correct. If we bear in mind the vicissitudes of climate and 
locality to which the sporangial cell may, under certain cir- 
cumstances, be subjected, we can readily understand, more- 
over, how increased or diminished sources of nutritive matter, 
dependent on those vicissitudes, may affect the produce of 
that cell towards either extreme. 
In Isthmia, a genus offering remarkable facilities for the 
detection of differences between the size of the old and the 
new valves of the frustules, after careful and oft-repeated 
examination, I have been quite unable to detect any differ- 
ences independent of the causes associated with the connecting 
zone to which reference has already been made. In this genus 
and in Biddulphia, the overlapping of the two rings of the 
connecting zones is more strikingly manifest, perhaps, than 
in any other forms, and the entire frustules are often of such 
magnitude as to enable us clearly to distinguish the contrast, 
