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SCIENCE 



[N. S. Vol. XL. No. 1026 



cultivated kinds have lost. The legend that 

 the seedlings of cultivated apples become 

 crabs is often repeated. After many in- 

 quiries among the raisers of apple seed- 

 lings I have never found an authentic case 

 — once only even an alleged case, and this 

 on inquiry proved to be unfounded. I 

 have confidence that the artistic gifts of 

 mankind will prove to be due not to some- 

 thing added to the make-up of an ordinary 

 man, but to the absence of factors which in 

 the normal person inhibit the development 

 of these gifts. They are almost beyond 

 doubt to be looked upon as releases of 

 powers normally suppressed. The instru- 

 ment is there, but it is "stopped down." 

 The scents of flowers or fruits, the finely 

 repeated divisions that give its quality to 

 the wool of the merino, or in an anahDgous 

 case the multiplicity of quills to the tail of 

 the fantail pigeon, are in all probability 

 other examples of such releases. You may 

 ask what guides us in the discrimination of 

 the positive factors and how we can satisfy 

 ourselves that the appearance of a quality 

 is due to loss. It must be conceded that in 

 these determinations we have as yet re- 

 course only to the effects of dominance. 

 When the tall pea is crossed with the 

 dwarf, since the offspring is tall we say 

 that the tall parent passed a factor into 

 the cross-bred which makes it tall. The 

 pure tall parent had two doses of this 

 factor; the dwarf had none; and since the 

 cross-bred is tall we say that one dose of 

 the dominant tallness is enough to give the 

 full height. The reasoning seems un- 

 answerable. But the commoner result of 

 crossing is the production of a form inter- 

 mediate between the two pure parental 

 types. In such examples we see clearly 

 enough that the full parental characteristics 

 can only appear when they are homozygous 

 — formed from similar germ-cells, and that 

 one dose is insufficient to produce either 



effect fully. When this is so we can never 

 be sure which side is positive and which 

 negative. Since, then, when dominance is 

 incomplete we find ourselves in this diffi- 

 culty, we perceive that the amount of the 

 effect is our only criterion in distinguishing 

 the positive from the negative, and when 

 we return even to the example of the tall 

 and dwarf peas the matter is not so certain 

 as it seemed. Professor Cockerell lately 

 found among thousands 'of yellow sun- 

 flowers one which was partly red. By 

 breeding he raised from this a form wholly 

 red. Evidently the yellow and the wholly 

 red are the pure forms, and the partially 

 red is the heterozygote. We may then say 

 that the yellow is YY with two doses of a 

 positive factor which inhibits the develop- 

 ment of pigment; the red is yy, with no 

 dose of the inhibitor ; and the partially red 

 are Yy, with only one dose of it. But we 

 might be tempted to think the red was a 

 positive characteristic, and invert the ex- 

 pressions, representing the red as RB, the 

 partly red as Br, and the yellow as rr. 

 According as we adopt the one or the other 

 system of expression we shall interpret the 

 evolutionary change as one of loss or as one 

 of addition. May we not interpret the 

 other apparent new dominants in the same 

 way? The white dominant in the fowl or 

 in the Chinese primula can inhibit color. 

 But may it not be that the original colored 

 fowl or primula had two doses of a factor 

 which inhibited this inhibitor 1 The pepper 

 moth, AmpJiidasys betularia, produced in 

 England about 1840 a black variety, then 

 a novelty, now common in certain areas, 

 which behaves as a full dominant. The 

 pure blacks are no blacker than the cross- 

 bred. Though at first sight it seems that 

 the black must have been something added, 

 we can without absurdity suggest that the 

 normal is the term in which two doses of 



