200 MANUAL OF ELEMENTARY ZOOLOGY 



between gametes derived from near relatives, or inbreed- 

 ing, is often found to be harmful — a fact which is well 

 known to breeders, and is expressed in the marriage laws 

 of many nations — yet in some animals it has no bad results 

 whatever. It is perhaps safest to suppose that the 

 advantages of conjugation, whatever they be, generally 

 accrue in the highest degree when the gametes are not 

 closely related. At the same time it must be remembered 

 that gametes belonging to different species, if they will 

 unite at all, nearly always produce a mule or hybrid which 

 is incapable of reproducing its kind. 



