432 COMPARATIVE ZOOLOGY 



odd, i.e., single, or two placed one behind- the other. 

 Recent creatures with feathers always have beaks. 

 Pigeons with short beaks have small feet; and those 

 with long beaks, large feet. The long limbs of the 

 hound are associated with a long head. A white spot 

 in the forehead of a horse generally goes with white 

 feet. Hairless dogs are deficient in teeth. Long wings 

 usually accompany long tail feathers. White cats with 

 blue eyes are usually deaf. A sheep with numerous 

 horns is likely to have long, coarse wool. Homologous 

 parts tend to vary in the same manner ; if one is diseased, 

 another is more likely to sympathize with it than one 

 not homologous. This association of parts is called 

 correlation of growth. 



6. Individuality 



It seems at first sight very easy to define an individ- 

 ual animal. A single fish, or cow, or snail, or lobster is 

 plainly an individual ; and the half of one such animal 

 is plainly not one. But when we consider animals in 

 colonies, like corals, it is not so easy to say whether the 

 individual is the colony or the single polyp. Is the tree 

 the individual, or the bud ? If we say the former the 

 colony what shall we say to the free buds of a hydroid 

 colony, living independent lives, and scattered over 

 square miles of ocean ? Are they parts of one individ- 

 ual ? If we choose the latter as our standard, we are in 

 equal difficulty ; for we must then call an individual the 

 bud of the Portuguese man-of-war, reduced to a mere 

 bladder or feeler, and incapable of leading an indepen- 

 dent life. We thus find it necessary to distinguish at 

 least two kinds of individuals -physiological individ- 

 uals, applying that name to any animal form capable 

 of leading an independent life ; and morphological indi- 

 viduals, one of which is the total product of an egg. 



