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genera of the group the two halves of the valve are precisely 
alike. 
The form and structure of the frustule being established, the parts 
capable of affording good distinctive marks for species were next ex- 
amined. All naturalists agree, that if these are taken from variable 
parts, they must be of less importance than if derived from those 
that are subject to little or no variation ; and that no observation 
can be relied on, of a permanent kind, when taken from parts known 
to change their appearance rapidly. Thus, the zone connecting the 
two valves of a diatom, which, from being a mere line, is understood 
to attain the whole breadth of the frustule in the course of twenty- 
four hours, has been deservedly rejected ; and hence it is to be feared 
that few or none of Dr Gregory’s species of “ Complex Amphorae,” 
which owe their peculiar appearance to it, will stand the test of diag- 
nostic characters. As the striae, costae, or furrows, are the same on 
both sides of the median line, and as the valve is folded, those at 
the back of the frustule must be seen through the medium of the 
surface nearer the eye, and crossing those belonging to it, so that ob- 
servations on these relate entirely to the accidental position the frus- 
tule happens to be in. This compels one to depend chiefly for es- 
sential characters — 1st, on the small portion that is seen between 
the median line and the apparent outline of the frustule ; and 2d, on 
the form of the frustule itself, previous to the siliceous connecting 
zone commencing the process of self-division. 
The author also stated his conviction that no certain conclusions 
could be drawn as to what was a new form or species from deposits 
or dredgings, on account of the impossibility of procuring the species 
in an isolated state, and consequently of studying them independently; 
the same species putting on very different aspects, and different spe- 
cies assuming the same aspect at particular stages of self-division. 
Microscopical differences are by themselves of little importance. 
To see is one thing, to understand and combine what we see, another : 
the eye must be subservient to the mind. Every supposed new spe- 
cies requires to be separated from its allies, and then subjected to a 
series of careful observations and critical comparisons. To indicate 
many apparently new species is the work of an hour, to establish 
only one on a sure foundation is sometimes the labour of months or 
years. In microscopical natural history as much scrutiny is requir- 
ed to prove a new form to be distinct from its allies as in chemistry 
