116 DUDLEY MEMORIAL VOLUME 



explanation in reinvasion on the part of one or both of the forms. The 

 obvious immediate element in the formation of species is, therefore, isolation, 

 and behind this are the factors of heredity, of variation, of selection, and 

 others as yet more or less hypothetical. The formation, through segregation, 

 of different breeds of sheep in the different countries of England, as noted by 

 Jordan and Kellogg (p. 82), seems exactly parallel with the formation of 

 species in nature. In like manner, the occasional development of breeds 

 arising from the peculiarities of individuals is possibly parallel with the 

 ''mutations" of the evening primrose. It seems to me probable, however, 

 that these mutations are phenomena of hybridism. Such breeds are the 

 Ancon sheep in Connecticut and the blue-cap Wensleydale sheep in Aus- 

 tralia. The hornless Hereford cattle lately established in Kansas is a case 

 in point. The "ontogenetic species" — groups in which many individuals are 

 simultaneously modified in the same way by like conditions of food or cli- 

 mate — show no permanence in heredity. Such forms, however strongly 

 marked, should, therefore, have no permanent place in taxonomy. The recent 

 studies of Mr. Beebe on the effects of moist air in giving dusky colors to 

 birds serve to illustrate the impermanence of the groups or subspecies char- 

 acterized by dark shades of color developed in regions of heavy rainfall. 

 These dark shades are not inherited and are not constant in the same in- 

 dividual if it is brought under new conditions. 



It may also be noted in passing that one cause of the potency of arti- 

 ficial selection 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 artificial selection, in isolation, no process of selection without 

 isolation can have any permanent result. For example, we know no way 

 of improving the breed of salmon, because the salmon we have selected for 

 reproduction must be turned loose in the sea, where they are at once lost 

 in the mass. 



New forms of gold fish, carp and other domesticated fishes can be made 

 easily by selection, 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 have termed geminate species — twin species — each one representing 

 the other on opposite sides of some form of barrier. In a general way, these 

 geminate species agree with each other in all the respects which usually dis- 

 tinguish species within the same genus. In all matters of selection and adap- 



