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HUGHES : INGLEBOROLGH. 
accompanying if not caused by environment. Another interesting 
line of research is where we find in one area a certain well-marked 
species or variety, and in another area, at the same time, similar 
or closely allied forms suggesting a contemporary modification 
for which we can sometimes find a cause in changes of sediment, 
of depth of water, or other conditions — though accident often 
mixes up the modified forms, so that, although their area of 
development and normal habitat may have originally been 
different, they arc now thrown together. 
What contribution can our student make towards the 
working out of these interesting questions ? Let him select 
any one horizon in which he sees a fair abundance of certain 
allied forms and collect all he can, being careful to look out 
for intermediate varieties ; let him arrange them to illustrate 
the gradual passage from one variety to another, and he will 
probably be rewarded by finding that what have been regarded 
as distinct varieties and species can, by the interpellation of 
intermediate forms, be run into one variable group. I have 
in the Sedgwick Museum many series of ammonites, gaster- 
opods, and brachiopods, arranged in continuous sequence, and 
passing, without any sensible break, through four or five or 
more species. We shall see as we go on how much this is wanted 
for Ingleborough. When our student has worked this out, 
then, and not till then, will he be in a position to appreciate 
the modification of species as time went on. On Ingleborough 
we have an unbroken sequence of some 1,500 or 1,600 feet of 
Carboniferous rocks within an area of very small extent, so 
that we can study the modification of species in time without 
taking account of their geographical distance apart. 
To make this sort of work possible the first essential is 
a map which must be based upon those differences of sedimenta- 
tion, lithological character, and structure to which the obvious 
features are due. For these stratigraphical features are easily 
recognised, and do not get altered continually with the pro- 
gress of investigation. Xew quarries, keener eyes, more hammers 
turn out fossils where they had not been found before, and 
our ideas as to the range of certain forms have to be modified 
from time to time, but a band of limestone, for instance, carefully 
