No. 567] EFFECT OF DISTRIBUTION ON SPECIATION 14V) 



gress in a definite direction, but the trend of variation or 

 progression may be different in one locality, and tend 

 towards a different result, from that of another locality. 

 Whether the evolution, usually in more or less divergent 

 directions, of segregated groups of individuals be looked 

 upon (1) as the accumulation of numerous slight varia- 

 tions which have a different average character in any two 

 portions of a species, as originally explained by Darwin 

 (1859, Chap. 4) or (2) purely as the result of natural selec- 

 tion, as argued by Wallace (1858), or (3) as the result of 

 a change in the average character of two portions due to 

 the uneven occurrence of mutations in the two portions, 

 a conclusion reached by Dewar and Finn (1909, p. 380), or 

 (4) as the result of orthogenetic evolutionary tendencies 

 inherent in the species and influenced by the environment, 

 as Eimer suggested (1897, Chap. 1), does not concern us 

 here, — the general tendency appears to be that two iso- 

 lated portions of a species as a general rule trend in 

 different directions, and diverge farther and farther as 

 long as they are isolated. 



It is assumed that the greater the length of time given 

 for the influence of isolation to be felt, the farther apart 

 are the two originally identical divisions likely to trend, 

 however the dissimilar evolution be interpreted. As 

 stated by Tower (1906), in speaking of the method of 

 evolution of the Chrysomelid genus Leptinotarsa, 



We can interpret the conditions found by any of the current 

 hypotheses; but explaining a condition by an hypothesis is not the 

 same as that the conditions found are evidence in support of an 

 hypothesis, although it is often so used. 



The existence of distinct variations, subspecies, and 

 ultimately species and genera, in isolated areas is a too 

 frequently observed phenomenon to be looked upon as 

 anything else than a self-evident truth, but that this should 

 necessarily be considered as supporting any particular 

 theory of evolution can not be argued. 



The profound results of prolonged isolation may be 

 observed in the fauna of some of our long- separated con- 



