24O COOK 



ALTERNATION OF GENERATIONS (METAGENESIS). 



In many animals and plants the usual method of propagating 

 new individuals by new sexual conjugations gives place to a 

 more or less regular alternation with generations which are 

 propagated vegetatively, or without a new conjugation. Among 

 the animals, such as the tunicates and plant-lice, the generations 

 which propagated vegetatively have a form different from those 

 which propagate by renewed conjugation. 



Alternation of generations, in the proper sense of the words, 

 occurs when the same species exists in two alternative forms, 

 and especially where the two forms have different methods of 

 propagation. The plant-lice furnish the most familiar example 

 of alternation of generations. We may suppose that, like other 

 insects, they were confined originally to normal sexual repro- 

 duction, but their evolution has been in the direction of smaller 

 size and simpler structure, and they have also developed the 

 power of multiplying for several generations by partheno- 

 genesis, the parthenogenetic generations being further distin- 

 guished by the absence of wings, and by being very short-lived. 

 At the end of the season winged insects of both sexes are pro- 

 duced, and normal fertilization and egg-laying ensues. 



No such alternation of sexual and parthenogenetic generations 

 is known to have arisen among plants, though a similar interpre- 

 tation might be placed upon the bamboos, for example, which 

 propagate vegetatively by the branching of their root-stocks for 

 a long series of years. Then all the plants of the species blos- 

 som, bear fruit and die, at the same time. Each sterile shoot 

 of the bamboo might be interpreted as parthenogenetic genera- 

 tion if compared with the sexually propagated generations of a 

 plant like Indian corn. 



METAMORPHOSIS. 



Among the insects in particular, and to a somewhat less de- 

 gree in many other animals (mollusca, Crustacea, batrachia, 

 fishes, etc.). pronounced changes of form and structure, some- 

 times very abrupt, take place during the life-history of each in- 

 dividual. Thus caterpillars change by metamorphosis into 

 butterflies, grubs into beetles, maggots into flies, tadpoles into 

 frogs, etc. 



