Dr. Wallich on the Rhizopods. 327 



In tlie case of the Difflugidai there is no anomaly. For, 

 although in the tests of the new forms to which I shall 

 hereafter have occasion to refer in detail, some singularly 

 striking characters become noticeable, there is, strictly speak- 

 ing, no complexity in their construction as imjparted to them 

 hy the onimal, but only a very exceptional character, which 

 carries with it indisputable evidence of not being the result of 

 inherited idiosyncrasy, but of the variable nature of the condi- 

 tions present in the medium in which the animal lives. This 

 view was strongly urged by me in my paper in the ' Annals ' 

 for March 1864, and in a previous paper in the same Journal 

 for Dec. 1863, in the following words: — "At the most, therefore, 

 mere modifications in the shape and proportionate quantities 

 of the organic and inorganic elements entering into the for- 

 mation of the shell, ought to be employed only in dis- 

 criminating between species." — Annals, June 1863, p. 452. 

 And again : — " Assuming from the facts which have been 

 advanced that the shape, materials, size, and colour of the 

 Difflugian tests furnish characters so conspicuously variable 

 as to yield no trustworthy criterion for even generic or even 

 true specific distinction, and recalling to mind once more that 

 the animal is in every instance specifically the same, it 

 appears to me impossible to arrive at any other conclusion 

 than that the whole of the subspecies, as well as their inter- 

 mediate varieties (widely though some of these seem to diff'er 

 from others in external features), have not only been derived 

 by direct descent from a single progenitor, but may still con- 

 tinue to be produced by direct descent from varieties which 

 become permanent * ; and may one and all still be produced 

 from a common archetype under the varying conditions to 

 which these lower forms of life are subject. 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 constituted, in obedience to local conditions and its 

 own requirements." — Annals, March 1864, p. 239. 



* " Permanent " only in the sense of being so as long as the conditions 

 underwhich the species or variety first became established remain unchanged. 

 When these conditions become gradually or suddenly modified, so do the 

 species or varieties, but only in those respects in which the conditions effect 

 a change in the animal itself, in its shelly covering, or in both combined. 

 Thus, a dry season or a flood, or extreme degrees of temperature in the 

 medium in which the animals live, scarcity or deterioration in the food- 

 supply, one and all bring about modifications which then tell on their 

 stability, their tendency to variation, or their extermination. This, in all 

 probability, is the reason why we so often find some special form we have 

 been accustomed to look for in a given locality, either replaced by 'a 

 varietal form or o'one altogether. 



