ONE-EGG TWINS IN FISHES 39 



fishes, salmon, trout, mackerel, perch, pike, and killifishes. 

 The trout is by all odds the favorite type and no less 

 than twenty-five separate reports of duplicity in trout 

 eggs have appeared. Some significance attaches to 

 the fact that the trout shows a higher incidence of 

 duplicity than other species. In the first place the 

 trout is the favorite game fish of the world and is more 

 commonly reared artificially in hatcheries than is any 

 other fish. This alone would account for the more 

 frequent observation of monstrosities of various sorts, 

 but would not account for the higher percentage of 

 double monsters and twins. It seems probable that the 

 trout, being a fish of the cold, pure, and thoroughly 

 oxygenated waters of streams and spring- fed lakes, 

 is more abnormally environed during development in 

 crowded hatcheries than would be most other fishes. 

 The key to the cause of twinning doubtless lies in this 

 circumstance, as we shall later attempt to show. 



Of the various authors who have contributed to our 

 knowledge of twinning in fishes the following, placed in 

 their chronological order, seem to me to deserve especial 

 attention: Lereboullet (1855 and 1861), Knoch (1873), 

 Rauber (1877, 1878, and 1879), Klausner (1890), Windle 

 (1895), Kopsch (1899), Schmitt (1901 and 1902), 

 Gemmill (1901 and 1912), Stockard (1921). 



MODES OF ORIGIN OF FISH TWINS 



Various classifications of fish twins have been given 

 by different authors. That of Gemmill (191 2) seems 

 the most satisfactory of those hitherto published. 



Whatever be the causation, we may recognize in vertebrates 

 generally, four somewhat different modes of origin, whether for 



