FUNDAMENTAL RELATIONS OF ANIMALS 93 



In other classes other phenomena of a similar character have been 

 observed, which bear a similar explanation. J. Miiller has most fully 

 illustrated the alternate generations of the Echinoderms; Chamisso, 

 Steenstrup, Eschricht, Krohn, and Sars, those of the Salpae; von Sie- 

 bold, Steenstrup, and others, those of certain Intestinal Worms. 



This alternate generation differs essentially from metamorpho- 

 sis, though some writers have attempted to identify these two proces- 

 ses. In metamorphosis as observed among Insects the individual born 

 from an egg goes on undergoing change after change, in direct 

 and immediate succession, until it has reached its final transforma- 

 tion; but however different it may be at different periods of its 

 life, it is always one and the same individual. In alternate genera- 

 tions the individual born from an egg never assumes through a suc- 

 cession of transformations the characters of its parent, but produces, 

 either by internal or external budding or by division, a number, 

 sometimes even a large number of new individuals, and it is this 

 progeny of the individuals born from eggs which grows to assume 

 again the characters of the egg-laying individuals. 



There is really an essential difference between the sexual repro- 

 duction of most animals and the multiplication of individuals in 

 other -^vays. In ordinary sexual reproduction every new individual 

 arises from an egg and by a regular succession of changes assumes 

 the character of its parents. Now, though all species of animals re- 

 produce their kind by eggs, and though in each there is at least a 

 certain number of individuals, if not all, which have sprung from 

 eggs, this mode of reproduction is not the only one observed among 

 animals. We have already seen how new individuals may originate 

 from buds, which in their turn may produce sexual individuals; we 

 have also seen how, by division, individuals may also produce other 

 individuals differing from themselves quite as much as the sexual 

 buds, alluded to above, may differ from the individuals which pro- 

 duce them. There are yet still other combinations in the animal 

 kingdom. In Polyps, for instance, every bud, whether it is freed 

 from the parent stock or not, grows at once up to be a new sexual in- 

 dividual; while in many animals which multiply by division every 

 new individual thus produced assumes at once the characters of 

 those born from eggs. There is, finally, one mode of reproduction 



