308 Plant Life and Evolution 



been found almost impossible to increase the per- 

 centage materially beyond this point, presumably 

 due to the fact that the constitution of the cells will 

 not permit of higher concentration of the cell 

 sap. It is probable that there are limits in size also 

 beyond which certain forms cannot go. It is hardly 

 likely that any amount of selection will enable the 

 gardener to grow pansies as big as peonies, or 

 cherries the size of oranges. 



Sports. Discontinuous variations have of late 

 attracted much attention, owing to the important 

 work of De Vries and his disciples. These discon- 

 tinuous variations, or " sports," have been long 

 known and have given rise to many garden varieties 

 of fruits and flowers. One of the best-known cases 

 is that of the nectarine, which is a sport from the 

 peach. The importance of De Vries' work, which 

 will be referred to presently somewhat more at 

 length, is his systematic study of the origin of these 

 sudden variations, and the demonstration that they 

 may be made permanent, where crossing is pre- 

 vented. 



ORTHOGENESIS 



It is believed by many biologists that variation is 

 often determinate, i.e., along definite lines, probably 

 adaptive in their nature. If the definition of deter- 

 minate variation, " Variation along special or par- 

 ticular lines of adaptation," is accepted, there can be 



