322 CONCLUDING CHAPTER 



shoots exactly resembling those of one or other of the 

 parents often appear upon the ' hybrid ' plants. 



It now appears that in Cytisus Adami the cells of the 

 two component species remain perfectly distinct, and 

 that its reproductive cells are always of the laburnum 

 type. In spite of the intimate association of the two 

 groups of cells which build up a common plant body, 

 the cells of Cytisus purpureus are unable to transmit 

 any hereditary influence to the cells of the laburnum, 

 and these give rise to offspring which are pure laburnum. 

 The epidermis of the ' graft-hybrid ' is said to consist 

 wholly of cells of the Cytisus purpureus type. It seems 

 fair to argue that if one species wrapped in the epi- 

 dermis of another receives no heritable influence what- 

 ever from its living integument, it is in the highest 

 degree unlikely that the germ cells will be able to 

 acquire transmissible modifications from an environ- 

 ment wholly external to the plant. The proof may 

 not be absolutely conclusive, but when it is combined 

 with all the other evidence pointing in the same direc- 

 tion, we think that the inheritance of acquired char- 

 acters may be disregarded as a practical factor in 

 evolution. 



Meanwhile the number of cases in which discon- 

 tinuity of inheritance can be shown to hold good is 

 constantly increasing, and the analysis of some cases 

 of supposed continuous variation into discontinuous 

 Mendelian factors has already been made. It may 

 be safely concluded that a very large part, if not the 

 whole, of evolution has taken place by the discon- 

 tinuous method. 



New little species Jordan's species arise, then, 



