342 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 Difflugidce. 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 



