MUSEUMS AND RESEARCH 73 



mouth : characters that may be regarded, for the most part, 

 as connected with differences of environment and of habits. 

 Sometimes the Char of neighbouring lakes are quite dissimilar. 



In the case of the Char physical isolation has followed 

 habitudinal segregation, and I think every systematic zoologist 

 recognises that physical isolation has often been a condition of 

 the evolution of species. A community may become split into 

 two or more by physical barriers that prevent intercommunica- 

 tion, or part may become separated from the parent stock by 

 migration into a new area ; under these circumstances geo- 

 graphical or representative species may arise, one form in- 

 habiting one area, its nearest relative representing it in another 

 usually an adjacent area. 



One cannot get a better idea of the effects of physical 

 isolation than by comparing the marine fishes of the Atlantic 

 coast of Central America with those of the Pacific coast. The 

 fishes of the two coasts are very similar, and a number of the 

 Atlantic species are represented on the Pacific side by forms 

 that can only be distinguished from them by a detailed com- 

 parison. In other words, taking the fishes of the Atlantic and 

 Pacific coasts together, a large proportion can be grouped into 

 pairs of closely related species, one member of each pair on 

 the Atlantic coast of Central America and the other on the 

 Pacific coast. 



There is good reason for believing that in Eocene times 

 there was a marine connection between the Atlantic and 

 Pacific oceans across what is now the Isthmus of Panama, and 

 that in the Miocene this connection came to an end, and there 

 can be little doubt that each pair of species is descended from 

 a parent species that was found on both coasts when the two 

 oceans were connected. 



I have given examples to show that habitudinal segrega- 

 tion, habitudinal segregation followed by physical isolation, or 

 physical isolation alone, may have been the conditions under 

 which new subspecies or species have arisen. 



It often happens that two or more species inhabit the same 

 area under circumstances that indicate their derivation in that 

 area from a single ancestral type : in such cases, whenever 



