8 GENETICS 



pointed out, accidental and quite outside the plan of 

 nature. 



In these cases it is easy to see the reason for "or- 

 ganic resemblance" between successive generations. 

 Parent and offspring are successive manifestations 

 of the same thing, just as the begonia plant, restored 

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

 tension of the original plant. 



Many modifications of the process of multiplica- 

 tion by fission occur, all of them, however, agreeing 

 in the fundamental principle that the progeny re- 

 semble the parents because they are pieces of the 

 parents. 



Thus the greening apple maintains its individuality 

 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. 



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 

 gemmules and statoblasts. These structures may 

 drop to the muddy bottom and live in a dormant 

 condition throughout 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 



