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THE AMERICAN NATURALIST [Vol. XL V 



confinement, when they were separated. After 89 hours the 

 female was killed, the remainder of her eggs placed on glass 

 slides, and fertilized with a decoction of the testes of other (one 

 or more?) males. If many of the eggs soon rotated within their 

 membranes this was taken as a sign of successful fertilization. 

 It will be noted that a different male from that employed for the 

 normal fertilization was necessarily employed, because the 

 original male had presumably lost his power to further fertilize. 

 The employment of different males introduces a possible error 

 into the results, for, if the male is heterozygous for sex determina- 

 tion, it is conceivable, as I have previously pointed out in review- 

 ing Hertwig's results, that in different individuals the sperm 

 may be differently affected in regard to its fertilization power. 

 At present we have no evidence to show that in male frogs such 

 differences exist, and it seems unlikely that such consistent results 

 as these of Hertwig and of Kuschakewitsch can be explained in 

 this way. An alternative view is, however, possible. If the 

 female is heterozygous for sex production, and in consequence 

 two kinds of eggs are produced, it may be that the female deter- 

 mining eggs are more injured by delay than are those of the other 

 class, the male-determining eggs. It becomes, therefore, impera- 

 tive to know what proportion of eggs were fertilized in these 

 experiments, Unfortunately this critical evidence is omitted 

 from Kuschakewitsch 's paper. He states that the death rate of 

 the tadpoles that emerge is low, but one looks in vain for informa- 

 tion relating to the number of eggs that were fertilized. There- 

 fore until this datum is forthcoming it is not possible to draw 

 any certain conclusions in regard to sex determination from the 

 evidence published by the author. 



T. II. Morgan. 



THE MUTATION THEORY 

 The publication of the first volume of DeVries's "Mutations- 

 theorie" in 1901, together with the rediscovery of Mendel's 

 principles, served to bring about a period of unprecedented 

 activity in the study of the problems connected with variation, 

 heredity and evolution. While the results of this decade of work 

 have probably raised as many questions as they have answered, 

 yet the period has undoubtedly been marked by advances of 

 the first importance, both in methods of investigation and in 



