1 68 THE EVOLUTION OF SEX. 



SUMMARY. 



1. Old theories of "ovists," " animalculists, '"'' and of the "aura 

 seminalis." 



2. Modern morphological theories incline to lay the whole emphasis 

 upon the nuclei. The conclusions of Hertwig and Strasburger are strongly 

 in favour of this view. The claims of the cell-substance and general proto- 

 plasm must not, however, be overlooked. Many facts, such as those demon- 

 strated by Boveri, show that the protoplasm is also important. 



3. Modern physiological theories of fertilisation are necessarily very 

 tentative. Sachs compares it to fermentation ; Rolph to mutual digestion. 

 To Weismann, the process appears quantitative rather than qualitative, so 

 far as the subsequent division is concerned. Half of the Keimplasma which 

 the ovum has surrendered with the second polar globule is restored by the 

 sperm-nucleus. The two nuclei are alike, and thus there is virtually no sex. 

 Protest against this. The male cell brings to the ovum a supply of char- 

 acteristic katastates. 



4. Uses of fertilisation to the species. I\Iany regard fertilisation as a 

 necessary rejuvenescence of the life of the species. Weismann criticises 

 this view, but his criticism must be read in the light of the researches of 

 iMaupas, who has shown that without conjugation the members of an 

 isolated family of infusorians eventually cease to feed and divide, passing 

 through stages of degeneration and senility to extinction. In this case, 

 conjugation is essential to the continued vitality of the species. According 

 to Brooks, fertilisation is an important source of variation ; according to 

 Weismann, it is really the sole source. 



LITERATURE. 



Works already cited. 



Hertwig, O. — Das Problem der Befruchtung, &c. ; Jenaische Zeitschrift 



flir Naturwissenschaften, XVIH., 1S85. 

 Maupas, E. — Comptes Rendus, 1886, 1SS7 ; and Archives de Zoologie 



experimentale, 1 888. 

 Strasburger, E. — Xeue Untersuchungen iiber den Befruchtungsvorgang 



bei den Phanerogamen, als Grundlage fiir eine Theorie der Zeugung. 



Jena, 1884. 

 Weismann, A. — (9//. (TzV., especially Die Bedeutung der sexuellen Fortpflan- 



zung flir die Selektions-Theorie. Jena, 1886. 



