428 COMPARATIVE ZOOLOGY 



from generation to generation ; but it can be accumu- 

 lated by choosing from a large number of individuals 

 those which possess any given variation in a marked 

 degree, and breeding from these. Nature does this by 

 the very gradual process of " natural selection " ; man 

 hastens it, so to -speak, by selecting extreme varieties. 

 Hence we have in our day remarkable specimens of 

 poultry, cattle, and dogs, differing widely from the wild 

 races. 



Sometimes we notice that children resemble, not their 

 parents, but their grandparents or remoter ancestors. 

 This tendency to revert to an ancestral type is called 

 atavism. Occasionally stripes appear on the legs and 

 shoulders of the horse, in imitation of the aboriginal 

 horse, which was striped like the zebra. Sheep have a 

 tendency to revert to dark colors. 



The laws governing inheritance are unknown. No 

 one can say why one peculiarity is transmitted from 

 father to son, and not another ; or why it appears in one 

 member of the,family, and not in all. Among the many 

 causes which tend to modify animals after birth are the 

 quality and quantity of food, amount of temperature 

 and light, pressure of the atmosphere, nature of the soil 

 or water, habits of fellow animals, etc. 



Occasionally animals occur, widely different in struc- 

 ture, having a very close external resemblance. Barna- 

 cles were long mistaken for mollusks, polyzoans for 

 polyps, and lamprey eels for worms. Such forms are 

 termed homomorpJiic. 



Members of one group often put on the outward ap- 

 pearance of allied species in the same locality ; this is 

 called mimicry. " They appear like actors or masquer- 

 aders dressed up and painted for amusement, or like 

 swindlers endeavoring to pass themselves off for well- 

 known and respectable members of society." Thus, 



