INTRODUCTION 11 



from a fragment of a begonia leaf, is simply an ex- 

 tension of the original plant. 



Many modifications of the process of multiplication 

 by fission occur, all of them, however, agreeing in the 

 fundamental principle that the progeny resemble the 

 parents because they are pieces of the parents. 



Thus the "greening" apple maintains its individu- 

 ality although coming from thousands of different trees, 

 because all of these trees through the asexual process 

 of grafting are continuations of the one original 

 Rhode Island greening tree grown by Dr. Solomon 

 Drowne in the town of Foster, nearly a century ago. 

 Western navel oranges all come, directly or indirectly, 

 from parts of one tree found near Bahia in Brazil. 



Again, certain fresh-water sponges and bryozoans, 

 quite unlike most of their marine relatives, keep a 

 foothold from year to year within their particular 

 shallow fresh-water habitats by isolating well pro- 

 tected fragments of themselves in the form of geimrndes 

 and statoblasts. These structures may drop to the 

 muddy bottom and live in a dormant condition through- 

 out the icy winter when it would not be possible for 

 the entire organism to survive near the surface. 



In order to meet the conditions imposed by winter, 

 however, these fragments have become so modified as 

 temporarily to lose their likeness to the parent genera- 

 tion, although readily regaining that likeness when 

 springtime brings the opportunity. The unity of 

 two succeeding generations, notwithstanding that it 

 may be interrupted by the temporary interposition of 

 something apparently different in the form of gemmules 



