PRINCIPLES OF THE GROWTH OF TREES. 15 



successive generations may intervene. Thus, for example, 

 the seed of a pear never produces an apple, these being dis- 

 tinct species; but it gives many different sorts of pears, which 

 are only varieties. So the apple produces innumerable varie- 

 ties, but it can never yield a pear, a quince, or a peach. 



The knowledge of the character of species, and their affini- 

 ties, would frequently prevent the blunders which grafters 

 commit, in trying I o make the peach grow on the willow or 

 butternuts 



While species repr )duce the same species, varieties do not 

 always reproduce t' ^mselves. Varieties have been called 

 incipient species. By slow and gradual changes, varieties may 

 in time give rise to other species. So it happens that whereas 

 the species may not directly change into another species, it 

 may gradually pass into another by successive variations. 

 But in the lifetime of a man, specific limits are rarely over- 

 passed. 



