64 



SCIENCE 



[N. S. Vol. XXIX. No. 732 



not only to Castle's hypothesis, but to the 

 whole Mendelian interpretation. 



The second hypothesis is that of Cor- 

 rens, which assumes the male to be a sex- 

 hybrid while the female is a homozygous 

 recessive. The fertilization formulas are 

 accordingly 



Egg 5 + spermatozoon 5 = zygote 22 ( female ) 

 Egg ? + spermatozoon (J = zygote (5)c? (male) 

 This conclusion is based on the following 

 beautiful experiments. Crosses between 

 monecious and diecious floAvering plants 

 show that the monecious character behaves 

 like a "unit character" which is recessive 

 to the diecious. If reciprocal crosses be 

 made between the monecious Bryonia alba 

 and the diecious B. dioica the results are 

 as follows: Female dioica crossed with 

 male alba gives all females. The reverse 

 cross gives half males and half females. 

 From the fact that all the offspring of 

 female dioica X male alba are females Cor- 

 rens concludes that all the eggs bear this 

 tendency, which dominates the monecious 

 character of the male parent. In the re- 

 verse cross the diecious character again 

 dominates, but in this case is derived from 

 the male parent. The appearance of the 

 two sexes in equal numbers must therefore 

 mean that half the pollen grains bear a 

 dominant male tendency and half a re- 

 cessive female. A similar result is reached 

 by Noll by experiments of a quite different 

 character on the hemp, but the proof seems 

 to me less cogent. 



Correns's experiments are of admirable 

 ingenuity and his results seem at first 

 sight to be open to but one conclusion. 

 His interpretation renders the hypothesis 

 of selective fertilization unnecessary; for 

 the chance fertilization of any egg by any 

 spermatozoon explains the numerical 

 equality of the sexes in the same way that 

 it explains the equal numbers of the two 

 classes of offspring of an ordinary Mende- 

 lian cross between a liomozygote recessive 



and a heterozygote. An obvious difficulty 

 at once appears, however, in the partheno- 

 ■genetic forms; for here the partheno- 

 genetic females must bear both tendencies, 

 since they, sooner or later, produce males 

 without fertilization. We need not enter 

 into Correns's suggestions in regard to the 

 aphids and phylloxerans, since they are 

 contradicted by the facts of the spermato- 

 genesis. In case of the bee, he adopts 

 Beard's supposition that there are two 

 kinds of eggs— sexual female-producing, 

 which require fertilization, and partheno- 

 genetic male-producing. In the latter the 

 original female tendency is replaced by the 

 activation or setting free of a male tend- 

 ency previously latent.^ A similar ex- 

 planation might be applied to the aphid, 

 phylloxeran or daphnid. But does not 

 such manipulation of the sexual tendencies 

 greatly weaken the force of the Mendelian 

 interpretation 1 To me it seems that if the 

 sexual tendencies may thus be shifted back 

 and forth between the active and latent 

 states, the interpretation loses most of its 

 explanatory value. 



Can Ave then explain the difficulty in 

 question by reversing Correns's hypothesis, 

 assuming the male to be the homozygote, 

 the female the heterozygote? This is the 

 hypothesis of Bateson, who further sug- 

 gests that different species or groups may 

 differ in respect to the sex that is homo- 

 zygous. The fertilization formulas now 

 become : 



Egg 5 -|- spermatozoon (^ = zygote 5(c?) (female) 

 Egg 1^ -\- spermatozoon |^ = zygote ^^ (male) 



But new and even more serious difficul- 

 ties now arise. If the male be homozygous 

 in the ordinary forms of insects, what 

 * To understand this it must be borne in mind 

 that Correns regards each " active " sexual tend- 

 ency (whether dominant or recessive) as accom- 

 panied by a "latent" (not to be confused with 

 a recessive) opposite tendency. Such a latent 

 male tendency in the female, upon becoming ac- 

 tivated, would dominate the female. 



