Classification and Distribution of Animals. 17 



have organs in a more concentrated condition than in 

 their aquatic allies. 



Parasitism. Some animals pass their lives within 

 or on the bodies of others, and this condition induces 

 striking alterations in structure. In some cases the 

 intruder collects its own food independently of his 

 host, being thus only indebted to him for house room ; 

 of this nature are the sponges which live rooted on 

 crabs, or the barnacles on the skin of the whale. The 

 second series of intruders are fellow commoners with 

 their hosts, feeding on the food which their entertainer 

 collects ; while in a third class the parasite is a pen- 

 sioner on the body of his host, feeding on his sub- 

 stance. Such forms are true parasites. 



In all these conditions there is a diminished 

 necessity for locomotion and for food-capture on 

 the part of the parasite ; so the organs of motion, of 

 sense, and of nutrition retrograde, but as the parasitic 

 condition involves difficulties in the continuance of 

 the species, the organs of multiplication are enor- 

 mously increased in size and complexity. 



Extension in Time. Species of animals have 

 limited ranges in time as well as in space, for they 

 are dependent on the constancy of physical conditions 

 for their specific longevity, and such alterations in these 

 as are constantly occurring will tend to extinguish 

 species ; hence the history of life in the past is a con- 

 tinual record of the dying out of types of life. 



