CRUCIAL INSTANCES 165 



explained on one only of these rival hypotheses ; . . . such a test 

 case is called a Crucial Instance" ] " One single circumstance, 

 which admits of one explanation only, is more decisive than a 

 hundred others which agree in all points with one's own hypo- 

 thesis, but are equally well explained on an opposite hypothesis." 2 

 Now, as regards the rival hypotheses of segregation and latency, 

 any number of cases have been recorded which furnish quite 

 decisive crucial instances ; but, though the experimental school 

 has piled one untested interpretation on another, though they have 

 explained alternative reproduction by segregation, segregation by 

 factors and determiners, and determiners by ferments, never yet 

 has any appeal been made to them. Ancestral traits may be 

 awakened experimentally by crossing domesticated varieties ; but 

 these are not the only occasions when they appear. Their reproduc- 

 tion by pure-bred individuals is far from uncommon. "Thus we 

 see that, in purely bred races [of pigeons] of every kind known in 

 Europe, blue birds occasionally appear having all the marks which 

 characterize C. livia" 3 The following is a very striking instance 

 of the reappearance of long latent ancestral characters in an 

 individual which not only reproduced them, but had previously 

 reproduced those of her own variety as well. Several like instances 

 have been recorded. " Mr Hewitt possessed an excellent Sebright 

 gold-laced bantam hen, which, as she became old, grew diseased in 

 her ovaria, and assumed male characters. In this breed the males 

 resemble females in all respects except in their combs, wattles, 

 spurs, and instincts ; hence it might have been expected that the 

 diseased hen would have assumed only those masculine characters 

 which are proper to the breed ; but she acquired, in addition, well- 

 arched tail sickle-feathers quite a foot in length, saddle-feathers on 

 the loins, and hackles on the neck ornaments which, as Mr Hewitt 

 remarks, ' would be held as abominable in this breed.' The 

 Sebright bantam is known to have originated about the year 1800 

 from a cross between a common bantam and a Polish fowl, 

 recrossed by a hen-tailed bantam, and carefully selected ; hence 

 there can hardly be a doubt that the sickle feathers and hackles 

 which appeared in the old hen were derived from the Polish fowl 

 or common bantam ; and we thus see that not only certain masculine 

 characters proper to the Sebright bantam, but other masculine 

 characters derived from the first progenitors of the breed, removed 



1 Welton, Manual of Logic, vol. ii. p. 102. 

 1 Ueberweg, Logic, Eng. Trans., p. 513. 

 8 Animals and Plants, vol. i. p. 206. 



