MAKING NJUW KINDS OF PLANTS 



445 



being in a state of continual slow change, are con- 

 stant. Professor de Vries, as the result of his own 

 experiments, believes that species are constant except 

 at certain mutation periods which may occur only 

 at long intervals, perhaps of some hundreds of years. 



248. Lamarck's Evening Primrose at the riglit, and one of the new species wliich has 

 arisen from it (Dwarf Evening Primrose) at the left. The bags are for the pur- 

 pose of excluding insects from the flowers which are being artificially pollinated. 

 Botanic Garden of the University of California. 



The differences between the new species and the 

 parent form may be great or small: it does not 

 matter which they are, if only they are fixed and 

 constant. Fig. 248 shows a striking difference in 

 appearance between the parent- species and one of 

 the new ones. This difference cannot be overcome 

 by culture; the tallest Dwarf Evening Primrose is 

 always much smaller than the shortest Lamarck's 

 Evening Primrose. 



