368 MR. G. S. WEST ON 
those species occurring in. deep bog-pools and the quiet margins 
of deep lakes, at which places these enemies abound, are possessed 
of a more complicated, and in many cases of a formidable, 
exterior. 
As regards the conditions favourable to variation among the 
members of this group of plants, it may be said that the occur- 
rence in large quantity of a particular species is most conducive 
to the production of deviations from the normal form. It may 
happen that in some localized spot an immense quantity of some 
particular species is occasionally produced by very rapid division, 
and in such a case some variation is almost always met with. 
Wallich was also of opinion that the prolific growth of large 
numbers of Desmids tended to produce variations, as in com- 
menting upon the Desmidiee of Bengal, he states? that 
“amongst the more common species a remarkable amount of 
divergence from the typical character is everywhere to be met 
with—a circumstance depending, no doubt, on those peculiarities 
of soil and climate which, in Lower Bengal, are so favourable 
to the exuberant and rapid development of the entire vegetable 
kingdom." Wide distribution and diversity of physical con- 
ditions may also tend to bring about the same result, but these 
factors exercise by no means so marked an effect on these lowly 
plants as on more highly organized plants and animals. Many 
Desmids have a very extensive and a few a world-wide distribu- 
tion, yet such species appear to have no apparent constitutional 
variation adaptive either to the requirements of the different 
climates in which they exist, or to the varying altitudes at which 
they occur. This may be chietly, or at least in part, owing to 
their aquatic mode of existence. 
The most numerous variations are to be found amongst the 
commoner and more widely distributed species, those occurring 
in rarer forms being, as a rule, either extremely slight or very 
abnormal. The majority of these variations appear to affect 
only the superficial characters—the warts, spines, striolations, 
granules, scrobiculations, &e., on the external or internal surfaces 
of the cell-wall. Some of them, however, are more important 
modifieations, being changes in the external form or symmetry 
of the plant; and yet others are variations in the structure and 
arrangement of the cell-contents. 
* G. C. Wallich, * Desm. of Lower Bengal," Ann. Mag. Nat. Hist. 1860. 
