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 "' is incorrect. 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 specific differences of these forms, Ijelong to the same class. We thus see that 

 natural selection has pla3'ed no part in the development of these characters of these 

 species. But tliis 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 'prcqnnquus-growp, such as it actually is, 

 is able to live and to fiourish, but it is not responsible for the splitting up of this 

 group into two or three species. 



The latter fact is entirely due to isohition. 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 tliis 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 ohsctmis 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 

 projyhiquns-gvoui:) has spHt 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 occu^jy neighboring areas." (See Ortmann, 19056, p. 127, Jordan, 

 Science, Nov. 3, 1905, p. 546, and Merriam, 1906, p. 248, et scq.) 



(c) C. hartoni. 

 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 



