EXCEPTIONAL PHENOMENA. 335 



always been so, but have, in wandering through space, 

 been entangled in our system and retained by the attrac- 

 tive influence of Jupiter, or one of the other larger planets. 

 We must then limit the statement of the law of uniform 

 direction to bodies which are derived from the original 

 constituents of the nebulous mass. 



Limiting exceptions occur most frequently in the natural 

 sciences of Botany, Zoology, Geology, &c., the laws of 

 which are almost wholly empirical. In innumerable in- 

 stances the confident belief of one generation has been 

 falsified by the wider observation of a succeeding one. 

 Aristotle confidently held that all swans are white 1, and 

 the proposition seemed true until not a hundred years 

 ago black swans were discovered in Western Australia. 



o 



At one time all the animal remains discovered in the 

 Scottish Old Red Sandstone were fishes or shells, until 

 at last a single small air-breathing reptile occurred oppor- 

 tunely to prevent any hasty conclusions 1 ". In zoology and 

 physiology we may expect a fundamental identity to exist 

 in the vital processes, but continual discoveries show that 

 there is no limit to the apparently anomalous expedients 

 by which life is reproduced. Alternate generation, fer- 

 tilization for several successive generations, hennaphro- 

 ditism, are opposed to all we should expect from 

 induction founded upon the higher animals. But such 

 phenomena are only limiting exceptions showing that 

 what is true of one class is not true of another. In 

 certain of the cephalopoda we meet the extraordinary 

 fact that an arm of the male is cast off and lives inde- 

 pendently until it encounters the female. 



Q ' Prior Analytics,' ii. 2, 8, and elsewhere. 

 r Murchison's 'Siluria' (1854), p. 254. 



