146 THE EVOLUTION OF SEX. 



SUMMARY. 



I. Reproduction is but more or less discontinuous growth. 



II. Sexual reproduction normally implies (a) special reproductive cells, 

 distinct from the bcdy ; {b) the dimorphism of these cells ; (r) their physio- 

 logical dependence,— the ovum being unproductive without the spermatozoon, 

 .and vice versa. 



III. The discoveries of Camerarius, Amici, Kolreuter, Sprengel, and others, 

 laid the foundations of our knowledge of sexual reproduction in plants. 



IV. The history of research on fertilization in animals well illustrates the 

 gradually increasing precision of scientific inquiry. 



V. The conjugation processes seen in Protozoa are of much importance in 

 suggesting the origin of differentiated fertilization. 



VI. The origin of fertilization may be traced through the following grades: — 

 {a) plasmodial union, (6) multiple conjugation, (r) ordinary conjugation, (d) 

 union of dimorphic cells, (e) fertilization of ovum by spermatozoon. 



VII. Both in plants and animals hybridization is often successful, but the 

 offspring frequently tend to be sterile. This, however, must not be exag- 

 gerated. 



LITERATURE. 



See the already noted works of Balfour, Van Beneden, Carnoy, Geddes, 

 Haddon, Hensen, Hertwig, McKendrick, Sachs, and Vines. 



For recent papers see Boveri, Th., Zellen Studien ; Jenaische Zeitschrift fur 

 Naturwissenchaften, 1887-88; Zoological Record, from 1886; and Journal of 

 Royal Microscopical Society. 



