No. 629] DARWIN AND HYBRIDIZATION 



539 



or in some degree sterile; if fertili/ed witli pollen from another (lower 



fertile; if fertilized with pollen from another individmd or variety of 



species they are sterile in all possible dej>Tecs. until nltei- sterility is 

 reached. We thus have a long series with absolute sterility at the 



ferentiated, and at the other end to their having been differentiated 



The questions wliicli Darwin raises in this eonnoetion 

 are as follows (p. 458) : 



1. Why the individuals of some species profit greatly, 

 others very little by being crossed. 



2. Why the advantages from crossing now seem to ac- 

 crue exclusively to the vegetative and now to the repro- 

 ductive system, although generally to both. 



3. Why some members of a species should be sterile, 

 while others are entirely fertile with their own pollen. 



4. Why a change of environment or of climate should 

 affect the sterility of self-sterile species. 



5. Why the members of some species should be more 

 fertile with the pollen from another species than with 

 that of their own. 



Eegarding the general matter of sterility in hybrids, 

 Darwin comments as follows: 



It is notorious that when distinct species of plains aro ciossiil they 



ber. This unproductiveness varies in different speri,^ up to sterility 

 so complete that not even an ein]>ly <-a]><u]o is f,>rnio.l ( lb. 403). 



It is also notorious that n<,t ..mK ilv par.-m ~|m.,-,..^. hnt the hybrids 

 raised from them are nion> or less >iciilr, aiul that th.'ir pollen is in a 



