No practical biologist interested in sexual reproduction would be led to work out the detailed consequences experienced by organisms having three or more sexes; yet what else should he do if he wishes to understand why the sexes are, in fact, always two?
                                                                                    
                                        
                                        
                                        
                                        
                                                                                
                                            
                                                
                                                    
                                                        "The Genetical Theory of Natural Selection". Book by Ronald Fisher, 1930.