12 PRINCIPLES OF PLANT CULTUBJE 



Pyrus being the genus to which the apple belongs, and 

 mains designating which species of the genus is meant. 



23. Crosses and hybrids. — We have seen that in 

 sexual reproduction, a union of male and female cells 

 is almost always essential (17). When these cells proceed 

 from two individuals of different varieties (21, 436), the 

 offspring is called a cross; when they proceed from in- 

 dividuals of different species, it is called a hybrid. At the 

 present time there is some confusion as to the definition 

 of the terms cross and hybrid. The term hybrid is now 

 frequently understood to mean offspring resulting from a 

 sexual union between individuals of any two unlike plants, 

 whether they are varieties or species. Hybrids are possible 

 only between closely allied species and are often incapable 

 of reproduction, in which case they are said to be sterile. 

 The mule, which is a hybrid between the horse and the ass, 

 is a familiar example of a sterile h}'brid. Sterile hybrids 

 are not uncommon in plants. A hybrid that is capable 

 of reproduction is called a fertile hybrid. 



Hybrids may resemble both parents about equally or 

 they may resemble one parent more than the other. They 

 sometimes differ materially from either parent. The 

 offspring of fertile hybrids are generally variable in pro- 

 portion as their parents were different from each other, 

 and this variability may continue indefinitely. 



24. The theory of evolution, now generally accepted by 

 naturalists, assumes that the higher plants and animals 

 have been gradually e\'olved from lower forms. The 

 explanation which seems to account for evolution most 

 satisfactorily is that of individual variation (18), and 

 adaptation through the principle that those individuals 

 possessing peculiarities best fitting them to endure the 

 adverse conditions of environment have survived and 



