No. 509] THE CATEGORIES OF VARIATION 275 



tion gave evidence of being recessive, but this was not 

 borne out by many other cases in which crosses be- 

 tween normal individuals produced polydactylous young. 

 Crosses of normal individuals both of which were of 

 polydactylous ancestry yielded a much higher per cent, 

 of polydactylous young than did crosses in which one 

 individual came from a normal breed, thus showing a 

 certain tendency in the blood towards polydactylism even 

 when it did not manifest itself by any outward mark. 

 Different males of the same amount of polydactylous 

 ancestry often showed great variation in the potency 

 with which they were able to transmit the anomalous 

 character. 



The evidence goes to show that we are dealing here 

 with a tendency which, whatever may be its basis, varies 

 continuously and not abruptly, although producing varia- 

 tions which, taken alone, would naturally be classed as 

 mutations. The extra toe is a new character, but the 

 polydactylous breed behaves neither like an elementary 

 species nor like a retrograde variety. The character fluc- 

 tuates to the vanishing point and even beyond (as shown 

 by crossing experiments with individuals of different an- 

 cestry) and shows varying degrees of fidelity of trans- 

 mission in different strains. Do we not have a condition 

 intermediate between the abrupt discontinuous variations 

 which breed true with great fidelity and ordinary fluctua- 

 tions ? It might be said that we have to do with a muta- 

 tion which fluctuates to -an unusual degree, although it 

 originally depended upon a sudden change in the germ 

 plasm; but the assertion would have no evidence to rest 

 upon. It might be said, on the other hand, that the varia- 

 tion is dependent on the undue activity of some of the 

 factors of normal development, an expression, for in- 

 stance of increased growth tendency in the part at a cer- 

 tain period, and that this tendency is kept from definite 

 expression until it reaches a certain strength, when it 

 manifests itself as a sudden variation. This conclusion 

 is warranted, I believe, not only by the great variability 



