208 LAND AND FRESHWATER 
required at all in some groups as in others; but where modification 
of outward form exists, and also changes in the various internal 
organs, as in these Indian Mollusca, then gradation in classification 
can, with advantage, be extended beyond that of genera and species. 
Characters seem to me like the strands of a plait made up of 
different colours, which are renewed from time to time, increased 
or decreased in number as they are worked into it; they run 
through a certain length of the plait, representative of time, each 
succeeded in turn by a new strand, the new character, which takes, 
as it were, the place of the other—yect not exactly, for each successive 
strand never holds the same position one to the other previously 
held; they are intermingled in every conceivable manner as the 
plait is extended—some are short, some long, some thick, others 
again fining away to mere threads. How very representative 
these threads or strands are of the altering character of allied forms, 
and how dimly, yet how distinctly, certain older characters are pre- 
sented to us on close examination of all their parts! In the land- 
mollusca the characters given below are all of importance and should 
all be separately looked at before any attempt is made to assign 
a species to its proper genus, and, finally, to classify the genera with 
any degree of accuracy. 
Characters. 
EXTERNAL. INTERNAL. 
A. Form of foot and its proportion in | @. Radula. 
comparison to the shell or the | a’. Jaw. 
mantle-lobes. b. Generative organs. 
A'. Mucous pore and other orifices. b'. Capreolus or spermatophore. 
B. Mantle-lobes, form of. c. Other organs, renal &e. 
B'. Texture of epidermis, pedal line, | d, Retractor muscle attachments, of 
and segmental groovings. buccal mass, tentacles, Ke. 
B". Tentacles. 
O. Shell. 
Enough is now known of the above in genera of Indian Mollusca, 
so far as I have been able to examine and treat of them in this 
work, to place a few in relative position, and as near as we can do 
so ina lineal arrangement, in or near which it will not be difficult 
to intercallate those that have yet to be examined. This may be 
considered somewhat premature work; but as time goes on other 
interests arise, bringing different work, and good sight (so much 
needed for this kind of investigation) may fail, so that it is best to 
bring observations together before making any fresh departure. 
