REPRODUCTION AND SEX 453 



each organism is both an egg-producer and a sperm-producer; 

 often, it must be noted, at different times, as in the "dichogamy" 

 of so many flowers. In such animals as those above mentioned, 

 there is cross-fertilisation in spite of hermaphroditism; in a few 

 other cases, such as the liver-fluke and some tapeworms, there is 

 self-fertilisation ("autogamy"). 



ADVANTAGES OF SEXUAL REPRODUCTION.— The biologist 

 must seek to explain the processes which bring about the separation 

 of parts, e.g. the formation of buds, as in Hydra, or the loosening 

 off of part of the body, as in Planaria, and also the multiplication 

 and liberation of eggs and sperms, as in most animals. We shall 



Fig. 56. 



Asexually Branching Marine Worm {Syllis ramosa). After Mcintosh. Two 

 heads (H) are shown, and there is a fusion of branches forming a network. 



return to these difficult physiological problems; but we give a prior 

 place to the question: What advantages has sexual reproduction 

 over asexual methods? Most plants and animals exhibit sexual 

 reproduction, though some retain asexual multiplication as well: 

 what justifies the sexual process? 



There is an advantage in the fact that a larger number of off- 

 spring can be started at once by the sexual method, and this usually 

 with relatively less drain on the resources of the body. That sexual 

 reproduction is fatal to the individual in many cases (e.g. butterflies, 

 eels, etc.) does not disprove the general proposition that sexual 

 multiplication tends to be, and generally is, a more economical 

 mode of reproduction than the asexual process. We use the term 

 "multiplication" as often as possible for all modes of increasing the 

 number of individuals, and correspondingly use the term "repro- 

 duction" when there is more or less distinct sexual differentiation. 



Although asexual multiplication occurs in some complex animals. 



