EVIDENCE FROM DIMORPHISM. 153 



tion, but also in habits of life, in such a way as to favour the 

 preservation of the race. Since the male sex generally has to take 

 a more active part in the acts of copulation and fertilization it is 

 comprehensible that the male form should differ more from the young 

 than the female which supplies material for the formation and 

 nourishment of the embryo and is charged with the care of the 

 progeny. Very frequently the male sex is capable of quicker and 

 more facile movements; in many Insects the male alone has the 

 power of flight, while the female remains without wings (fig. 97). 

 In the strife which the males of similar species have to wage for 

 the possession of the females, those individuals which are most 

 favoured by their organization (in respect of strength, capability for 

 motion, prehensile organs, beauty, organs for production of sound, 

 etc.) will prove the conquerors; while those females which possess 

 properties especially favourable to the prosperity of the offspring will 

 best fulfil their task. 



At the same time variations in the duration of development, in 

 the mode of growth and structure, may in a more passive way be 

 favourable under the special conditions of life of the species. The 

 secondary sexual characters may sometimes acquire such importance 

 as to lead to essential and deeply engrained modification of the 

 organism, and to a true sexual dimorphism (males of Rotifera with no 

 digestive tube, dwarfed males of Bonellia, Trichosomum crassicauda). 



It is a significant fact that dimorphism of sex reaches its highest 

 extreme in parasites. In many parasitic Crustacea (Siphonostoma) 

 such extreme cases, in which the large shapeless females have lost 

 the organs of sense and locomotion, and even segmentation, while 

 the males are small and dwarfed, are connected by numerous inter- 

 mediate forms; and the circumstances which have operated as the 

 cause of this sexual dimorphism are not far to seek. The influence 

 of favourable conditions of nourishment which parasites enjoy does 

 away with the necessity of rapid and frequent locomotion, increases 

 in the female the capacity of producing reproductive material, and 

 brings about such an alteration of form that the power of locomotion 

 is diminished and the organs of movement atrophy and may com- 

 pletely vanish. The body acquires an unwieldy, shapeless character 

 in consequence of the enormous size of the ovary which is filled with 

 eggs, and throws out outgrowths and processes into which the ovaries 

 project, or else acquires an unsymmetrical saclike form. The seg- 

 mentation is lost and the limbs degenerate; the slender moveable 

 abdomen which, when the animal was free-swimming, was an essen- 



