156 THE EVOLUTION OF Sr:X. 



SUMMARY. 



1. Reproduction is hut more or less discontinuous growth. 



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

 distinct from the body ; (/-') the dimorphism of these cells ; [c) their 

 physiological dependence, — the ovum being unproductive without the sjier- 

 matozoon, and vice versa. 



3. The discoveries of Camerarius, Amici, Kolreuter, Sprengel, and 

 others, laid the foundations of our knowledge of sexual reproduction in 

 plants. 



4. The history of research on fertilisation in animals well illustrates the 

 gradually increasing precision of scientific inquiry. 



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

 in suggesting the origin of differentiated fertilisation. 



6. The origin of fertilisation may be traced through the following 

 grades : — {a) plasmodial union, (/>) multiple conjugation, {i) ordinary con- 

 jugation, {(/) union of dimorphic cells, {c) fertilisation of ovum by sperma- 

 tozoon. 



7. I>oth in plants and animals hybridisation is often successful, but the 

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

 exaggerated. 



LITERATURE. 



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

 Haddon, Ilensen, Ilertwig, M'Kendrick, Sachs, and Vines. 



?'or recent papers see Boveri, Th., Zellen Studicn ; Jenaische Zf.-itschrift 

 fiir Naturwissenschaften, 1887-88; Zoological Record, from 1886; antl 

 Journal of Royal Microscopical Society. 



