GENERAL DISCUSSION. 17 



It is evident that every fertilized aphid egg must contain the female 

 sex character, since it invariably produces a parthenogenetic female. 

 The possible combinations of parthenogenetic, sexual female, and male 

 embryos cited in No. 7 of the Summary of Results, page 13, show that 

 in all probability every parthenogenetic female is capable of pro- 

 ducing sexual female progeny; in other words, the female character is 

 dimorphic, if one may so describe it. It gives rise under some condi- 

 tions to the development of a parthenogenetic female, under other 

 conditions to a sexual female. Now it remains to be determined 

 whether (i) each fertilized egg also contains the male character re- 

 cessive (Castle, '03) , or (2) whether only a part of the eggs contain the 

 male and female characters as alternates, while the remainder have 

 the female character in both the maternal and paternal series of chro- 

 mosomes. The former condition male and female characters in all 

 eggs rwould imply maturation of the winter eggs according to the 

 law of chances and meeting in fertilization of only eggs and sperma- 

 tozoa containing different sex characters (Castle, '03). The latter 

 condition might be the result of such a differential maturation of the 

 eggs that all should contain only the female sex character before fer- 

 tilization. The sex possibilities of the fertilized egg would then be 

 determined by the entrance either of a spermatozoon containing the 

 male character or of one containing the female character. An egg 

 fertilized by a spermatozoon containing the female character would 

 then produce a line of aphids consisting only of parthenogenetic 

 females and sexual females. An egg fertilized by a spermatozoon con- 

 taining the male element would be capable of producing a succession 

 of generations consisting of parthenogenetic females, sexual females 

 and males. This is just what we find (Summary, No. 7), but the same 

 conditions could occur if all the fertilized eggs contain both male and 

 female characters. 



The fact that in some species (Oenothera aphid No. I and the white 

 rose aphid) the two sexes are entirely separate in the sexual genera- 

 tion, and that in the six species in which embryos of both sexes are 

 found together there are always many individuals that contain only 

 female embryos, suggested the possibility that there might be in each 

 species entirely separate lines from the fertilized egg through the par- 

 thenogenetic generations to the sexual generation, some lines being 

 capable of producing males, others not. The only way to test this 

 point would be to isolate parthenogenetic individuals of a species, 

 either directly from the fertilized egg or from somewhat widely sepa- 

 rated hosts, and examine the sexual generations from each. If many 

 individuals from some lines of descent should give only sexual 



