4. INTRODUCTION. 
Nassa and had obtained a knowledge of the whole of the species either 
figured or described in it, they could not by this plan follow the in- 
tricate passages revealed by the study of variation. It is not by the 
intimate knowledge of the species themselves that these facts are 
elicited, but it depends upon a knowledge of the innumerable inter- 
mediate forms which diverge from them in every direction as to how 
these deductions are to be drawn. The conchologist and I are dealing 
with two distinct subjects, intimately connected, but differing mate- 
rially in the result obtained; the one is confined to the number of the 
shells figured and described, while the other wishes to embrace 
every variety occurring in creation, most of which are neither 
figured nor described. 
When the whole series is spread out, in lines in their trays these 
shells forcibly remind us of the people standing round a race-course 
when the horses are running. The different sizes in height and breadth, 
the different coloured clothes, the marked differences in form and 
feature, and the various casts of countenance, all exhibit, in pro- 
portion to the difference in the size of the objects compared, just 
such an amount of variation as we find in the varieties of the 
Nassz. 
The lines of descent from the largest to the smallest forms are 
often distinctly indicated, notwithstanding the large number of 
variable shells intervening between the first form and the last. 
Shells may be selected, in series, that will show a clear line in 
the descending order from Nassa glans, Linn., to Nassa incrassata, 
Miill., variety glaberrima ; but if these varieties are examined in the 
order in which they appear to fall naturally, then we find that an 
off-shoot takes place at the junction of the varieties of Nassa mucro- 
nata, A. Adams ; another with Nassa Marratii, Smith ; a third with 
the Nassa gaudiosa, Hinds, etc. Another of these lines may be 
started with the Nassa trifasciata, Gmel.; but in this case, instead 
of: tracing smooth shells, as we did in the last, the shells would 
pass into coronated, costated, and cancellated forms before returning 
to a similar small form to that with which we started. Most of the 
costated varieties such as Nassa nodifera, Powis, the cancellated 
varieties such as Nassa marginulata, Lam., and the elongated series 
