OTHER THEORIES OF SPECIES-FORMING. 3 6 5 



numerary fourth digit on one of the hind feet of one of nine young 

 produced by a certain pair of guinea-pigs. "Neither of the parents 

 had such a digit, nor had I ever heard of the existence of such a 

 character before, either in any of the wild Caviedae or among domes- 

 ticated cavies or guinea-pigs. Further, I have been able to find 

 no reference to such a thing in the literature of the group, though 

 I have several times since found this same mutation in other herds 

 of guinea-pigs. The mother of my four-toed pig never produced 

 another similar individual, though she was the mother in all of 

 thirty young. The father, however, who sired in all 139 young, 

 had five other young with extra toes, but these were all by females 

 descended from himself, so that it seems certain that the mutation 

 had its origin in this particular male. By breeding together the four- 

 toed young and selecting only the best of their offspring, I was able 

 within three generations to establish a race with a well-developed 

 fourth toe on either hind foot. This race was not created by selec- 

 tion, though it was improved by that means." Castle also had 

 another mutation appear in a second family of guinea-pigs. "A 

 few individuals were found to have hair about twice as long as 

 that of their parents and grandparents. Intermediate conditions did 

 not occur. Long-haired individuals mated together were found 

 to produce only long-haired young, so that a new breed was already 

 fully established without the exercise of any selection." Casey 

 (Science, N. S., Vol. XXII, p. 308, 1905) presents a number of 

 facts touching the sudden appearance of certain molluscous genera 

 in early Eocene strata, and in certain Lower Oligocene rocks, which 

 seem to be evidence for the mutations theory. "At least, the 

 mutation theory is evidently the best that has been advanced to 

 account for these known facts." Scott (Science, N. S., Vol. XXII. 

 pp. 271-282) attempts to make out a case for the mutational origin 

 of nine kinds of North American birds that, because of their rarity 

 and the obscure character of the records of their occurrence, are 

 mostly rather puzzling to ornithologists. (They are all included 

 in the "hypothetical list" of the American Ornithologists' Union 

 Check-List.) But this attempt is robbed of much significance by 

 Allen's critical discussion of it (Science, N. S., Vol. XXII, pp. 431- 

 434, 1905). Morgan (Harper's Monthly Mag., Vol. CVI, p. 478) 

 refers to the "japanned" turkeys, a kind of bronze-shouldered aber- 

 ration that appears occasionally in flocks of turkeys, as "mutations." 

 These turkeys are called attention to by Darwin ("Variation of 

 Animals and Plants Under Domestication," Vol. I, p. 305). Indeed, 

 more cases of such mutations are referred to and described by 

 Darwin himself than by all those who have attempted recently to 

 adduce examples, for the support of the mutations theory, of an 



