504 MEMOIRS OF THE CARNEGIE MUSEUM 



that these species have originated out of rather slight variations from tlie original 

 forms. If this is evident in so small a territory, further investigations onl}' can em- 

 phasize this, for additional material can onl}' bring these forms closer together. (See 

 Merriam, 1906, p. 257.) 



3. Formation of Species hij Isolation, as Exemplified by the 

 Pennsjjlva nia 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 

 "isolation" 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 

 (/. c, p. 190). 



Of these the last one, Sepcuration 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 spied cs ; but since separation alwa3's 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 insufficienc}' 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," /. c, p. 190.)*' That it may occur under several forms 

 is amply demonstrated by the Pennsylvania crawfishes, and some f(,)rm 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. 



*=■ " Barriers" are not necessary. Merriam, 1906, p. 249, tliinlis tliat the existence of sharp barriers is necessary for 

 isolation; where such are absent lie prefers to use the term "divarication." Possibly the terra " habitudiual segrega- 

 tion," introduced by Gulick, 1905, p. 49, and 53 el seq. would be appropriate. 



