No. 494] 



THE LAW OF GEMINATE SPECIES 



75 



potency of artificial select ion among domesticated animals 

 or cultivated plants is that such selection is always 

 accompanied by segregation. The latter is taken for 

 granted in discussions of this topic and hence its existence 

 as a factor is usually overlooked. While poultry or 

 pigeons can be rapidly and radically changed by arti- 

 ficial selection, in isolation, no process of selection without 

 isolation is likely to have any permanent result. For 

 example, we know no way of improving the breed of 

 salmon, because the salmon we have selected for repro- 

 duction must be turned loose in the sea, where they are 

 at once lost in the mass. 



New forms of gold-fish and carp can be made easily 

 in domestication, because these fishes can be kept in 

 aquaria or in little ponds, but new forms of mackerel or 

 herring are beyond the control of man and the species 

 actually existing have been of the slowest creation, their 

 origin lost in geologic times. 



One of the most interesting features of "Jordan's law" 

 is the existence of what I may term geminate species- 

 twin species— each one representing the other on oppo- 

 site sides of some form of barrier. In a general way, 

 these geminate species agree with each other in all the 

 respects which usually distinguish species within the 

 same genus. They differ in minor regards, characters 

 which we may safely suppose to be of later origin than 

 the ordinary specific characters in their group. Illustra- 

 tions of geminate species of birds, of mammals, of fishes, 

 of reptiles, of snails, or of insects, are well known to all 

 students of these groups, and illustrations may be found 

 at every hand. 



Each island of the West Indies, which is well separated 

 from its neighbors, has its own form of golden warbler. 



ing Sea has its own species of fur seal. 



One of the most remarkable cases of geminate species 



haS its own 

 genera of D 



