1918] on Gamete and Zygote L^S9 



There are examples of quantitative changes in zygotes and gametes 

 by which unions or divisions of equal and similar materials .are 

 effected. Analogy may be instituted between such unions and 

 divisions and those produced by physical means in various forms 

 of wave-motion.* The pattern, for instance, on Grevy's Zebra may be 

 said, almost without extravagance, to be the upper octave of that on 

 the Mountain Zebra. 



Divisions of differentiation or segregation among gametes commonly 

 distribute elements received as already dissimilar in fertilization. In 

 original variation^ by some irregularity in the process of division, 

 dissimilarity must be introduced. Lotsy has doubted whether real 

 proof of such contemporary variation exists, but cases apparently 

 incapable of any other interpretation can be adduced. A profusion 

 of new varieties has in modern times been produced, for instance, in 

 both Primula sinensis and the Sweet Pea, though all attempts to cross 

 these plants have failed. In such examples, moreover, there is clear 

 evidence of new gradational forms arising as rarities after much inter- 

 crossing of varieties. The appearance of a continuous variation 

 leading to extremes is illusory. Jennings, in the comparable instance 

 of Brosophila, has argued that since almost every gradation from 

 a pale red eye to a white eye now exists, the process of variation should 

 be regarded as continuous. But in Drosophila, as in the Sweet Pea, 

 the extremes came first, and intermediates have arisen by a process of 

 fractionation, representing almost certainly irregularities in the 

 perfection with which segregation has been accomplished. Such 

 irregularities may be compared with the various degrees of impurity 

 with which liquids incapable of mixing may be decanted from each 

 other. 



On the other hand, it is true that no satisfactory evidence of any 

 new positive ingredient being introduced, whether into zygote or 

 gamete, is yet before us. 



[W. B.] 



♦ Cf. W. Bateson, " The Problems of Genetics,"' 1913, chap. iii. 



