242 Dr. G.C. Wallich on the Extent and Causes of 
points equally distant from the common centre or archetype of 
the whole than as clustering round those points, for reasons 
which have been already given, but which it is perhaps desirable 
now briefly to recapitulate. 
As repeatedly stated, the entire series of Difflugian tests are 
constructed by animals which, with no known exception, are 
specifically identical. A priori, therefore, there is nothing im- 
probable in the supposition that the entire series, in their earliest 
condition—that is to say, when the chitinoid exudation, of which 
the test is entirely composed at that period, first makes its ap- 
pearance around the sarcoblast—are identical in form. But we 
have no need to assume the fact, for observation would seem to 
prove it. If this be granted, only one other condition is essential 
to the establishment of the doctrine that the entire series, not 
only in time past may have descended genetically from a single 
progenitor, but, what is of far greater moment, that they may 
still continue to be developed from a typical form common to all, 
—this condition being that the chain of transitional varieties 
should be so unbroken, and that there should be such a radiation 
of characters at every step, as to render it impossible to detect a 
character or set of characters which can be said to belong exclu- 
sively to a solitary form. 
Now this is precisely what observation teaches us when we 
study forms obtained from a sufficiently wide geographical range. 
For, as has been already shown, we not only then find that the 
unimportant gaps sometimes discernible between contiguous 
varieties are bridged over, but, if we note the differences in the 
external conditions by which the organisms are surrounded, we 
are able, generally speaking, to trace a relation between the 
varietal form and the agencies which have affected it. 
But, although there is good reason to believe that these con- 
clusions are correct, we must never lose sight of the fact, that 
under those peculiarities in the physical conditions which are 
inherent in every geographical area, and must therefore be 
regarded as constant in their operation, the repetition of certain 
varietal forms, to the exclusion of others, must also be constant. 
In this sense, but this sense only, can species and subspecies be 
established amongst the Difflugide. The animal does not vary; 
but it modifies the architecture of its habitation and the mineral 
material of which that habitation is in a great measure constructed, 
in obedience to local conditions, and in the manner best fitted to 
meet its requirements. 
It only remains for me to state that the universal distribution 
of these organisms, the ease with which they are obtained, their 
simplicity of structure, and the comparatively small number of 
types into which they seem to resolve themselves render them 
