No. 4.] CROSSING OF PLANTS. 37 



hybrids, for species have been bred away from each other 

 in the ability to cross. If, therefore, there is no advantage 

 for nature to hybridize, we may suppose that there would 

 be little advantage for man to do so ; and there would be no 

 advantage for man did he not place the plant under conditions 

 different from nature, or desire a different set of char- 

 acters. We have seen that nature's chief barriers to hybrid- 

 ization are total refusal of species to unite, and entire or 

 comparative seedlessness of offspring. We can overcome 

 the refusal to cross in many cases by bringing the plant 

 under cultivation ; for the character of the species becomes 

 so changed by the wholly new conditions that its former 

 antipathies may be overpowered. Yet it is doubtful if such 

 a plant will ever acquire a complete willingness to cross. 

 In like manner we can overcome in a measure the com- 

 parative seedlessness of hybrids, but it is very doubt- 

 ful if we can ever make such hybrids completely fruitful. 

 It would appear, therefore, upon theoretical grounds, that in 

 plants in which fruits or seeds are the parts sought, no good 

 can be expected, as a rule, from hybridization ; and this 

 seems to be affirmed by facts. It is evident that species 

 which have been differentiated or bred away from each other 

 in a given locality will have more opposed qualities or powers 

 than similar species which have arisen quite independently 

 in places remote from each other. In the one case the 

 species have likely struggled with each other until each one 

 has attained to a degree of divergence which allows it to per- 

 sist ; while in the other case there has been no struggle between 

 the species, but similar conditions have brought about similar 

 results. These similar species which appear independently 

 of each other in different places are called representative 

 species. Islands remote from each other but similarly 

 situated with reference to climate very often contain repre- 

 sentative species ; and the same may be said of other regions 

 much like each other, as eastern North America and Japan. 

 Now, it follows that, if representative species are less opposed 

 than others, they are more likely to hybridize with good 

 results ; and this fact is remarkably well illustrated in the 

 Kieffer and allied pears, which are hybrids between repre- 

 sentative species of Europe and Japan ; and I am inclined to 



