68 



SCIENCE 



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



than as a possible guide to inquiry. There 

 are many reasons for suspecting that it 

 does not reach the root of the matter. One 

 of them is the failure to account for the 

 significance of the Y-element, which is as 

 characteristic of the male sex, when it is 

 •present, as is the double X-element of the 

 female. Another is the possibility, which 

 is perhaps a probability, that other factors 

 than the chromosomes may play an essen- 

 tial role in sex-determination. The data 

 do not yet allow us to draw a positive con- 

 clusion on many of the detailed questions 

 of this kind. But our ignorance in regard 

 to these more specific problems does not 

 alter the fact that the cytological evidence 

 has revealed a visible mechanical basis for 

 the production of males and females in 

 equal numbers and irrespective of external 

 conditions; and this, I venture to think, 

 constitutes a real and important advance 

 in the investigation of the general problem 

 of sex. 



Vn. THE SEX-RATIO IN RELATION TO THE 

 CTTOLOGICALi BASIS OF SEX-PRODUCTION 



"We are thus led, finally, to the question 

 of the sex-ratios as they appear in the light 

 of the foregoing conclusions. It is well 

 known that different species often exhibit 

 characteristic differences in the ratio of 

 males to females; and this fact has been 

 urged by some writers as an argument 

 against the existence of an intrinsic and 

 uniform mechanism of sex-production and 

 against the specific assumption that sex is 

 transmitted as a Mendelian character. The 

 cytological facts seem to me, on the con- 

 trary, to offer the most valuable suggestions 

 for an understanding of the variations of 

 the sex-ratio. This appears from a consid- 

 eration of the extreme case where all the 

 fertilized eggs produce the same sex, as in 

 the aphids, daphnids and the like. A com- 

 plete explanation of these cases seems to be 

 given by the discovery that only the female- 



producing spermatozoa are functional. 

 May we not here find a clue to the explana- 

 tion of less extreme departures from the 

 equal ratio shown in other forms? It is 

 probable that the suppression of the male- 

 producing spermatozoa in the aphids and 

 phylloxerans was gradually brought about, 

 and was connected by intermediate stages 

 with the usual condition in which both 

 classes of spermatozoa are equally func- 

 tional. Stevens finds that in the aphids 

 all degrees of inequality exist between the 

 two classes of spermatocytes, though none 

 of the male-producing class seem to give 

 rise to spermatozoa. It seems reasonable 

 to suppose that such a condition has fol- 

 lowed one in which only a part of the male- 

 producing class became impotent or degen- 

 erate. Owing to the enormous number of 

 the spermatozoa, such a partial impotence 

 of this class would produce no noticeable 

 effect on the sex-ratio until it had pro- 

 ceeded very far. Sooner or later, however, 

 the proportion of males from fertilized 

 eggs would be reduced, and finally extin- 

 guished. Such a process would lead to the 

 extinction of the species were it not for a 

 compensatory parthenogenetic production 

 of males, such as, of course, exists in cases 

 where all the fertilized eggs produce fe- 

 males. 



As bearing on this question I may recall 

 the well-known fact that among the flower- 

 ing plants a certain proportion of the pollen 

 grains are often impotent, sometimes in a 

 definite ratio. Correns, for example, finds 

 that in Mirabilis longiflora there are three 

 impotent to one functional; in M. jalapa 

 the ratio is four to one. Such facts are 

 most suggestive in their bearing on the 

 whole question of sex-ratios, and the possi- 

 bility of their alteration by external agents. 

 Since the two classes of spermatozoa differ 

 in nuclear constitution it is highly probable 

 that they differ in respect to their meta- 



