454 COMPARATIVE ZOOLOGY 



(4) There is more or less variation even between closely 

 related animals. Two individuals from the same litter, 

 for instance, always differ somewhat from each other, as 

 well as from all their relatives, in shape, size, vigor, in- 

 telligence, and other qualities. Human beings, domes- 

 ticated or wild animals, birds, shells, or insects are never 

 so much alike that differences between individuals of the 

 same kind cannot be detected. Wings and tails of birds 

 of the same species have been found in some individuals 

 to be twenty per cent longer, and in others as much 

 shorter, than the average. Variations, then, are by no 

 means necessarily minute, but may be considerable in 

 amount. Nor is variation confined to structure alone, for 

 it may also affect habits. Chimney swifts built their 

 nests in hollow trees before the country was settled. A 

 New Zealand parrot which, before the occupation of the 

 island by Europeans, lived on honey, insects, and fruits, 

 began to pick at meat and skins hung up to dry by the 

 settlers, and thus acquired a taste for flesh. During the 

 past fifty years its carnivorous propensities have increased 

 to such an extent as to lead the bird to attack living sheep. 

 So destructive has it become that stringent measures have 

 been taken for its extermination. In animals under do- 

 mestication variation is the rule. The numerous breeds 

 of cattle, horses, swine, fowls, pigeons, dogs, cats, rabbits, 

 canary birds, and in fact of all domesticated animals, have 

 been derived from a few ancestral forms. Breeders have 

 taken advantage of peculiarities arising by variation ; and 

 by a process of selecting and breeding only from those 

 individuals which show the peculiarity, have finally suc- 

 ceeded in fixing it more or less permanently, so that the 

 young of these animals may possess it in an even more 

 exaggerated form than their parents. In nature varia- 

 tions occur to such an extent, particularly in large and 

 dominant genera, as to give rise to many doubtful species, 



