220 FOUNDATIONS OF BIOLOGY 



generations known as metagenesis occurs, which, thougn it 

 differs from, recalls the conditions which obtain in plants. 



Alternation of asexual and sexual methods of reproduction, 

 attended by more or less difference in structure of the indi- 

 viduals of the generations, is fairly widespread among the 

 Invertebrate groups, particularly in forms which have 

 adopted a parasitic mode of life. Frequently the life his- 

 tories are exceedingly complicated: several asexual, sexual, 

 and parthenogenetic generations succeeding one another in 

 response to the exigencies imposed by adaptation to a life 

 within another animal or series of animals. 



It is clear from such life histories that the conception of 

 special germ cells early set aside, as it were, from the somatic 

 cells must not be taken too literally. The same point is 

 emphasized by the power exhibited by plants and animals 

 in restoring parts lost by mutilations of one kind or another. 

 Among many plants, pieces of the root, stem, or, in special 

 cases, of the leaf may give rise to individuals complete in every 

 respect. Until the middle of the eighteenth century this was 

 considered a property peculiar to plants, and accordingly 

 soon after Hydra was discovered experiments were made to 

 determine whether the organism was a plant or an animal. 

 Specimens were cut into several pieces and it was found that 

 each piece developed into a complete Hydra. This result, 

 from the ideas of the time, should have led to the conclusion 

 that Hydra is a plant, but additional characteristics were ob- 

 served which outweighed all other considerations. Accord- 

 ingly Hydra was recognized as an animal with the power of 

 replacing lost parts. (Fig. 120.) 



Since the classic work on Hydra the power of regeneration 

 has come to be recognized as a fundamental property of all 

 animals. It is exhibited to the greatest degree among the 

 lower animals while in the higher Vertebrates it is confined 



