74 Mutations and Evolution. 



selection. It seems reasonable to suggest that the so-called death- 

 feigning instinct in many insects probably originated in the same 

 way. So far as I know, this is the first suggestion of a mutation 

 in an instinct. It would be selected in those insects in which such 

 action would save their lives. 



The hornless condition in cattle is another mutation of much 

 interest. Horns are known from the palaeontological record to 

 have undergone a gradual progressive development in various 

 groups of Mammals, while in the polled breeds the horns have 

 apparently been suddenly lost through a mutation. Punnett 1 has 

 suggested that the hornless condition may have co-existed in a 

 species along with the gradual development of horns. Since the 

 polled condition is dominant, however, at least in cattle, the 

 recessives would all be devoid of a factor for hornlessness. 

 And since the horns are a highly serviceable and necessary weapon 

 of defence in the wild species, it seems very probable that the 

 original hornless type would be stamped out by selection as soon as 

 horns developed far enough to become a valuable weapon. This 

 would take place whatever the causes of the progressive develop- 

 ment of horns. There are no traces of hornless cattle before the 

 historic era, although Herodotus describes the domestic cattle of 

 the Scythians as hornless. In the last two centuries many hornless 

 varieties have arisen and the origin of some polled breeds is known.* 

 The hornless mutation is in a sense a reversion as regards that 

 character, and as such is comparable with peloria in flowers. The 

 polled Hereford breed originated from a mutant at Atkinson, 

 Kansas in 1889. 3 Being dominant, it cannot be supposed to have 

 been present in the germplasm before its external appearance. 



An interesting and little known work by Bonavia (1895), while 

 containing some unacceptable ideas, devotes a chapter to 

 monstrosities as probable factors in the creation of species. 

 Among fishes are cited the sword-fish Histophorus gladius with its 

 upper jaw prolonged, Hemiramphius and various others with the 

 lower jaw prolonged; also Zygoena the hammer-headed shark, 

 which may have originated monstrously by a projection of both ocular 

 regions. A hairless condition is normal in certain races of Chinese 

 and Mexican dogs. It occurs as an anomaly in horses cattle and 

 dogs, and hairlessness in man " may have occurred all of a sudden 



< Mendelism. MacMillan. 



1 See MacDonald and Sinclair, 1882. History of polled Angus cattle. 



* Walter, H. E. Genetics, MacMillan, 1913. 



