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 allied 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 
(Pl. XV. figs. 3,3 v & Pl. XVI. fig. 24)*, stead 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+. 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 
erowth. But there are no grounds for such an assumption, 
since the minute tests occur wherever Difflugie 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. 1{, is made to assume either of those represented 
in the surrounding figures 2, 3, 4, & 5, according as the earliest 
* When noticing this variety of Diffugia (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, im 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. 
+ It may be mentioned that the most minute recognizable tests of Dif- 
flugia measure about +,';5th 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 »3,,th of an inch. 
+ 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 composition 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. 
