LECTURE XXX 



INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUC- 

 TION, AND THEIR CONSEQUENCES 



The separation of the sexes exists even among the Protozoa — Conditions determin- 

 ing the occurrence of Hermaphroditism — Tape-worms, Cirrhipeds — Primordial males 



Advantages of parthenogenesis — Alternation with bi-sexual generations — In Gall- 



-vvasps — In Aphides — Cross-fertilization secured in plants — Self-fertilization is avoided 

 whenever possible — The mechanism of fertilization and the mingling of germ-plasms 

 must be clearly distinguished from one another — Cases of jjersistent self-fertilization^ 

 The effects of inbreeding compared with those of parthenogenesis— The effect of purely- 

 asexual reproduction — In sea-wracks — In lichens and fungi — In cultivated plants — 

 Degeneration of the sex-organs — Summar5\ 



We have seen that continued inbreeding must make the germ- 

 plasm monotonous, and therefore unplastic as regards the requirements 

 of adaptation. Accordingly, we found that the gametes of many 

 unicellulars are so constituted that they only possess a power of 

 attraction for gametes of a different lineage, not for those of their 

 own stock. Among multicellular organisms the most intense mode 

 of inbreeding is to be found in the uninterrupted self-fertilization of 

 hermaphrodites : in such cases the monotony of the germ-plasm 

 must reach extreme expression more readily than in the case of 

 •ordinary inbreeding. We can thus understand why, in the scale 

 •of organisms, there is such an early occurrence of gonochorism, the 

 separation of the species into male and female individuals. Even 

 .among unicellular plants or Protophytes this occurs occasionally, as 

 it does in the A'orticellids among Infusorians. 



In the Metazoa and Metaphyta the separation of the sexes finds 

 •emphatic expression ; it is absent from no important group, and 

 in many, such as, for instance, among the Vertebrates, it has become 

 the absolutely normal condition, with hardly any exception. But 

 in many divisions of the animal and plant kingdoms hermaphroditism 

 also plays an important part, as, for instance, in terrestrial snails and 

 in flowering plants. 



Obviously the sexual adaptations of a species are definitely 

 related to the conditions of its life, and, though Nature's endeavour 

 :to prevent inbreeding and to secure cross-fertilization is evidenced 

 ;by the occurrence of separate sexes in such a multitude of forms 



