1888-89.] The Duke of Argyll on Bodies of Organic Origin. 53 
“ correlation,” which is the highest of all, namely, the adjustment 
of our perceptive faculties to the indications of external nature. 
In the first place, then, we must hear in mind the nature of the 
animals whose traces are in question. They are animals absolutely 
wanting both in external and in internal skeleton. Nothing that 
constitutes in most other animals the fossilisable elements of 
structure, has any existence in them. Soft animals, which have 
a shell, themselves decay, hut their shells remain, and thousands 
of extinct mollusca belonging to the past ages of organic life are in 
this way preserved to us with the utmost minuteness of detail. 
In this very rock, as it occurs in Sutherland, the serpulites discovered 
by Macculloch represent, in a silicified condition, the calcareous 
shells of some species of annelid, which lived, as some species now 
do, in an external shelly tube. But large classes of other marine 
worms or annelids have no such shells. They are entirely composed 
of tissues extremely soft, liable to rapid corruption after death, and 
incapable of being fossilised, except in the form of mere casts. 
On the other hand, we have to recollect that these naked an- 
nelids perform a work in the soft material of future rocks which 
is altogether peculiar to themselves — a work which arises out of 
their relation to peculiar sources of food, and for the performance 
of which they are endowed with very special and very wonderful 
powers. It is their function to bore into, or perforate, all kinds 
of marine sediment, both those which are in course of being laid 
down, and those even, in many cases, which have been long indurated 
into the hardest rocks. Confining our attention to those species of 
naked annelids which burrow in soft sands, clays, or mud, we find 
that there are many different genera which affect different kinds of 
sediment, and make burrows with some characteristic differences. 
Some line their tubes or tunnels with calcareous matters which are 
in the nature of a shell, and might be preserved like any other shell. 
But this is not the case with those annelids which are most common 
and abundant upon our existing shores. At every ebb tide upon 
our coasts, where there is sand or clay or mud, we see the surface 
covered with worm castings, and the presumption is that these are 
the living representatives of the primeval annelids which performed 
the same work in the earliest ages of which we have any record in 
the rocks. 
