216 COMPARATIVE ZOOLOGY. 



This is one of the most universal facts in nature. Ev- 

 ery development ends in diversity. All know that no 

 two individuals of a family, human or brute, are abso- 

 lutely alike. There are always individual differences by 

 which they can be distinguished. Evidently a parent 

 does not project precisely the same line of influences upon 

 each of its offspring. 



This variability makes possible an indefinite modifica- 

 tion of the forms of life. For the variation extends to 

 the whole being, even to every organ and mental char- 

 acteristic as well as to form and color. It is very slight 

 from generation to generation ; but it can be accumulated 

 Dy 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 



O / / 



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 then 

 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 struct- 



