508 MEMOIRS OF THE CARNEGIE MUSEUM 
flourishing. In other words, the ‘“selectional value” of this character is practically 
at the zero-mark. This demonstrates again that the conception of “ natural selec- 
tion ”’ as “selection of the fittest” isincorrect. With regard to fitness there are many 
characters which are entirely indifferent, and this is one of them. The absence or 
presence of a rostral keel, and of tubercles in the case of the female annulus, the 
other specifie differences of these forms, belong to the same class. We thus see that 
natural selection has played no part in the development of these characters of these 
species. But this does not imply that selection has had nothing to do with the evo- 
lution of these species, on the contrary this factor has always acted, and if these 
characters had not been fit to survive, the species would not have been able to sur- 
vive. Natural selection (in the modified sense, according to Pfeffer, see Ortmann, 
1896, p. 176). resulted in the fact that the propinquus-group, such as it actually is, 
is able to live and to flourish, but it is not responsible for the splitting up of this 
group into two or three species. 
The latter fact is entirely due to isolation. In the present case the isolation was 
in effect only during a short period in the past, but it was enough to differentiate 
several species. At the present time there is a tendency to undo this effect. These 
species are beginning to mingle again. But this process has not yet progressed far, 
and for several reasons will very likely be slow in future. It is hard to say what 
the outcome will be, whether we shall have a hybrid form, or whether one will sup- 
press the others. (C. obscwrus is the most advanced form, and also seems to be 
slightly more vigorous than the others. Thus it may finally overrun them and 
crowd them out, unless it is in turn conquered by a still more vigorous from, C. 
rusticus, advancing from the southwest. 
From the above discussion we see that whatever may have been the processes 
of variation and of natural selection, or independently of what we may think 
of the possibility of the inheritance of acquired characters, the fact that the 
propinquus-group has split up into species is solely due to isolation, which in this 
case is strictly topographical. We have here three forms with identical ecological 
habits, in which topographical isolation is evident, illustrating the rule that “closely 
allied species occupy neighboring areas.” (See Ortmann, 1905), p. 127, Jordan, 
Science, Nov. 3, 1905, p. 546, and Merriam, 1906, p. 248, et seq.) 
(c) C. bartont. 
This species is morphologically well isolated from the other Pennsylvanian 
species, and also has peculiar ecological habits. Being found all over the state it 
necessarily comes into contact with all the other species and is often found associated 
