320 CONCLUDING CHAPTER 
shoots exactly resembling those of one or other of the 
parents often appear upon the ‘ hybrid’ plants. 
It now appears that in Cyttsus 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 Cyttsus 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—Jorddn’s species—arise, then, 
