202 SCIENCE PROGRESS. 



i. When the normal form of a species (Grundart) is crossed 

 with a gradually formed local race of the same species, the re- 

 sult is a series of intermediate forms. 



2. When the normal form is crossed with a sporadic aber- 

 ration, the result in many cases is that the issue divides itself 

 sharply between the normal form and the sport, intermediate 

 forms being absent. 



Hence, according to Standfuss, the process of species- 

 formation must be gradual ; for when two distinct species 

 are crossed, the issue does not split up into the two parental 

 forms as in the case when one parent is a suddenly formed 

 aberration. On the contrary, the behaviour of the issue of 

 two distinct species is very similar in kind to that of a 

 species crossed with a local race or variety which is being 

 gradually established by the accumulation of slight changes. 

 It would seem therefore that although an aberration or sport 

 may be perpetuated by inheritance, it can never acquire 

 distinct specific rank. No doubt however it may, if se- 

 lected, eventually replace the original form of the species. 



To the foregoing considerations it may here be added 

 that these sporadic colour-aberrations seem to have many 

 points of resemblance with the colour- varieties in domestic 

 animals, such as the " lemon-and-white" and "tricolour" of 

 the Basset hounds referred to above (p. 199, note), or the 

 well-known tortoiseshell, tabby and black of cats. The 

 fact that these domestic varieties exist side by side in the 

 same race and even in the same litter, and that true inter- 

 mediates are rare or absent, seems to suggest that they 

 originally appeared as sports, and that their perpetuation 

 has been ensured or favoured by artificial selection, just as, 

 if Standfuss is right, the dark aberrations of P. monacha and 

 A. behdaria are being perpetuated and multiplied by natural 

 selection. Finally, the well-known case of the "otter sheep" 

 (Darwin, Variation of Animals and Plants under Domesti- 

 cation, 1868, vol. i., p. 100), where also intermediates are 

 said to be absent, and other instances on record, show that 

 the phenomenon of sharp division between types in the off- 

 spring is not confined to crosses between colour-varieties of 

 the same race, but occurs in other kinds of aberration as 

 well. 



F. A. Dixey. 



