220 Dr. G. C. Wallich on the Extent and Causes of 



is surrounded it may ultimately assume the shape of either of 

 the varieties W, X, Y, or Z. 



Of course, it is not only possible, but highly probable, that 

 the type of the mature variety may, in the greater number of 

 forms, be reproduced in the progeny ; but, be this as it may, 

 there can be no doubt of the fact that, with a solitary exception, 

 specimens are to be found in every habitat, the characters of 

 which overlap, so to speak, those of the most closely aUied va- 

 rieties to such a degree as to prove that there exist no true 

 specific limits between them ; whilst it is interesting to observe 

 that the exceptional form referred to, namely, Difflugia spiralis 

 (PI. XV. figs. 3, 3 tJ & PI. XVI. fig. 24)* instead of disproving the 

 fact advanced, confirms it, as shall hereafter be shown, inasmuch 

 as the recurved shape assumed by the test would seem to be pro- 

 duced by a purely mechanical agency which is only able to exert 

 influence upon it when it approaches maturity. 



It is also deserving of notice, as bearing upon the probable 

 existence of only a single distinct specific type in Difflugia, that 

 the number of mature tests in which the globular form prevails 

 falls immeasurably short of the number of young tests exhibiting 

 that shape f. This may, no doubt, be accounted for on the 

 supposition that the greater proportion of this the prevailing 

 form of young test may perish before they advance further in 

 growth. But there are no grounds for such an assumption, 

 since the minute tests occur wherever Difflugiee are most abun- 

 dant and the conditions for existence may therefore be assumed 

 to be the most favourable; and, as already stated, every gradational 

 variety is traceable even from this early stage. It seems much 

 more likely, therefore, that the globular figure, as shown in 

 Plate XV. fig. IJ, is made to assume either of those represented 

 in the surrounding figures 2, 3, 4, & 5, according as the earliest 



* When noticing this variety o{ Difflugia (Annals, June 1863, p. 451) 

 as having been observed by me in England for the first time, I was not 

 aware that it had been detected so long ago as the year 1815 by M. Leclere, 

 and accordingly gave it the provisional name of D.proteiformis, var. septi- 

 fera, in allusion to the dithalamous tendency evinced by its test. I am glad 

 to find that I am supported by Perty in the view that it is a " monstrosity " 

 of the form named, and not a distinct specific type. 



t It may be mentioned that the most minute recognizable tests of Dif- 

 flugia measure about xfrrirth of an inch in diameter, whilst the sarcoblasts 

 of th's genus, when extruded under pressure from the test so as to be 

 examined separately, average about ^VsTrth of an inch. 



X In order to make the characters referred to in the text more readily 

 intelligible to my readers, all references connected with the subject of 

 varietal modification in shape are made to the outline-figures in Plate XV. ; 

 whilst those bearing on the comjjosition and general outward features of 

 the tests are made to the detailed figures of the same forms given in 

 Plate XVI. Exceptional references will be expressly indicated. 



