40 THE EVOLUTION OF SEX. 



SUMMARY. 



1. The epoch at which the sex is finally determined is variable in different 

 animals, and diverse factors operate at successive epochs. 



2. Theological and metaphysical theories of sex have preceded the 

 scientific ; observation and statistics have been resorted to before experi- 

 ment ; and over 500 theories in all have been set forth. 



3-6. That there are two kinds of ova is still for the most part an assump- 

 tion ; that the entrance of more than one spermatozoon normally occurs, 

 and is a determining factor, is erroneous. Thury's emphasis on the age of 

 the ovum when fertilised is probably justified ; while Hensen extends this 

 notion to the male element as well. 'I'he age of the parents is probably only 

 of secondary import, and the law of Hofacker and Sadler is not confirmed. 



7, 8. Theories of "comparative vigour " and the like must be dismissed ; 

 while Starkweather's theory of the relative superiority of either sex, and of 

 the influence of this on the sex of the offspring, requires further analysis. 



9, 10. Darwin's position contains nothing novel, and has been superseded 

 by Diising's synthetic treatment and explanation of the self-regulating 

 numerical proportion of the sexes. 



10. From this point, after a note on the similar sex of "true" twins, 

 we pass to the experimental data and constructive treatment. 



LITERATURE. 



Berner. — Hj. Om Kjonsdannelsens Aarsager, En biologisk Studie (with 



numerous references). Christiania, 1883. 

 Darwin, C. — The Descent of Man, Chap. VIII. London, 1871. 



The Variation of Animals and Plants under Domestication. Lond. 



D USING, C. — Die Regulierung des Geschlechtsverhaltnisses bei der 



Vermehrung der Menschen, Thiere, und Pflanzen. Jena, 1884 ; or, 



Jen. Zeitsch. f. Naturw., XVII., 1883. 

 Geddes, p. — As before. 

 Hensen, V. — Physiologic der Zeugung. Hermann's Handbuch der 



Physiologic, Bd. VI., pp. 304, with references to Ploss, Schultze, &c. 



Leipzig, 1 88 1. 

 PIis, W. — Theorien der geschlechtlichen Zeugung. Arch. f. Anthropologic. 



Bde. IV. -VI. 

 Hofacker. — Ueber die Eigenschaften, welche sich bei Menschen und 



Thieren auf die Nachkommen vererben. Tubingen, 1828. 

 Laulani^, F. — Comptes Rendus, CI., pp. 593-5. 1885. 

 RoLPH, W. H. — As before. 



Roth, E. — Die Thatsachen der Vererbung (historical). Berlin, 1885. 

 Pfluger, E. — Ueber die das Geschlecht bestimmenden Ursachen und die 



Geschlechts-verhaltnisse der Frosche. Arch. f. d. ges. Physiol. XXIX. 

 Sadler. — The Law of Population. London, 1830. 

 ScHLECHTER. — Ucbcr die Ursachen welche das Geschlecht bestimmen. 



Rev. f. Tierheilkunde. Wien, 1884. Biol. Centralblt., IV., pp. 627-9. 

 Starkweather. — The Law of Sex. London, 1883. 

 Stieda. — Das Sexual Verhaltniss bei Geborenen. Strasburg, 1875. 

 Sutton, J. B. — General Pathology. London, 1886. 



Thury. — Ueber das Gesetzder Erzeugungder Geschlcchter. Leipzig, 1863. 

 Wappceus. — Allgemeine Bevolkerungs-Statistik. Leipzig, 1861. 



