504 MEMOIRS OF THE CARNEGIE MUSEUM 
that these species have originated out of rather slight variations from the original 
forms. If thisis evident in so small a territory, further investigations only can em- 
phasize this, for additional material can only bring these forms closer together. (See 
Merriam, 1906, p. 257.) 
3. Formation of Species by Isolation, as Exenvplified by the 
Pennsylvania Crawfishes 
I have repeatedly maintained that the whole process of evolution in nature which 
ends in the formation of ‘‘ species,” and which, consequently, may be called “ origin 
of species,” is not subject to one single factor alone, such as ‘natural selection ” or 
“Gsolation”’-or “mutation” but that it is absolutely necessary that several factors 
work together. (See Ortmann, 1896, p. 188 et seq.) Indeed none of these factors 
is new, and they have been discussed by various writers, but generally too much 
value has been attributed to one or the other of them to the detriment of the rest. 
I have insisted, on the contrary, that four factors are equally necessary to form 
species, namely: 1, variation; 2, inheritance ; 3, natural selection ; 4, separation 
(e=sps 90): 
Of these the last one, Separation or Isolation, is the one which forms species. To 
this is due the fact that the whole mass of organic beings to-day is divided up into 
a large number of units, which we call “species.” If it had never existed or acted 
the process of evolution would have gone on nevertheless, but the organic world 
would not consist of species; but since separation always has acted, species are pres- 
ent. This does not imply that species should be everywhere well-marked. This 
process is going on all the time, and in many cases it is not yet finished, and thus 
it may be difficult sometimes to say whether a particular form is to be regarded as 
a species or not; but, as a rule, our inability to declare positively that a certain form 
is a species is only due to the insufficiency of our knowledge. 
Separation (or isolation) should not only be conceived of in its broad topograph- 
ical and climatic aspect, but is, as I have always maintained, largely also ecological. 
(See “bionomic separation,” /. ¢., p. 190.)° That it may occur under several forms 
is amply demonstrated by the Pennsylvania crawfishes, and some form or other of 
isolation is evident in every case without exception. Both topographical and eco- 
logical separation are recognizable in our material, while climatic separation is 
not observed on account of the insignificant differences of climate in the region 
investigated. 
85 
“ Barriers’’ are not necessary. Merriam, 1906, p. 248,’thinks that the existence of sharp barriers is necessary for 
isolation ; where such are absent he prefers to use the term ‘‘divarication.’’ Possibly the term ‘‘habitudinal segrega- 
tion,’’ introduced by Gulick, 1905, p. 49, and 53 et seg. would be appropriate. 
