Classification and Distribution of Animals. 7 



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 

 nost, 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 in space, their specific 

 longevity depending on the constancy of the physical 

 conditions to which they are adapted, hence the 

 changes constantly occurring in these tend either to 

 extinguish or alter specific forms, and in the history 

 of life on the earth the old types have been constantly 

 dying out, and new types better adapted to their 

 environments survive. In any locality there is a 

 general resemblance between successive faunae, though 

 each usually shows a progress in specialisation on its 

 Dredecessor. 



