PLANT AFFINITIES 221 



would combine with a species of mint. One 

 member of the rose family may cross with an- 

 other blackberry with raspberry, let us say, or 

 quince with apple; and in the same way dif- 

 ferent species of oak may interbreed; but the 

 combination of apple or blackberry with any 

 species of oak is unthinkable. 



Similarly, I have been able to cross peach with 

 almond, and almond with plum, and plum with 

 apricot; also apple with quince, and quince with 

 pear. Stone fruit with stone fruit, and seed 

 fruit with seed fruit but not seed fruit with 

 stone fruit. 



In a word, the possibility of cross-fertilization 

 between species is conditioned on a certain close- 

 ness of relationship, which we speak of as affinity. 



This is a matter of actual genetic relationship. 

 All members of the rose family, for example, 

 have branched from the primal ancestral stem at 

 a period much more recent than that at which 

 the common ancestor of the present-day apple 

 and rose and blackberry branched from the 

 primal stock of, let us say, the oaks. 



In the broadest view, there is a cousinship 

 between all species of plants; just as there is 

 relationship between all the twigs of an actual 

 tree. But the species of an existing genus may 

 be likened to twigs on a single branch; other 



