ANIMAL PEDIGREES 231 



to secure the ribs, and this might lead to the 

 selection i.e., the survival, of an article that in other 

 and even in more important respects was manifestly 

 inferior to the average. 



So is it also with animals : the survival of a form 

 that is ideally inferior is very possible. To animals 

 living in profound darkness the possession of eyes 

 is of no advantage, and forms devoid of eyes would 

 not merely lose nothing thereby, but would actually 

 gain, inasmuch as they would escape the dangers 

 that might arise from injury to a delicate and 

 complicated organ. In extreme cases, as in animals 

 leading a parasitic existence, the conditions of life 

 may be such as to render locomotor, digestive, 

 sensory, and other organs entirely useless ; and in 

 such cases those forms will be most in harmony 

 with their surroundings which avoid the waste of 

 energy resulting from the formation and maintenance 

 of these organs. 



An excellent illustration of this downhill progress 

 is afforded by the Rhizocephala, a curious group of 

 parasitic Crustaceans, of which the genus Sacculina 

 is perhaps the best known member. The adult 

 Sacculina is found as a soft shapeless bag, an inch 

 or so in length, attached to the under surface of the 

 tail of a crab by a fleshy stalk which, passing 

 through the skin of the crab, spreads out within it 

 into a complicated system of branching tubular roots 

 by which the parasite sucks up the juices of the 

 crab, on which it depends for food. As regards 

 structure, the Sacculina in its fully developed form 

 is little more than a bag of eggs enclosed in a 



