Chap. XIX. SUMMARY OF THE FOUR LAST CHAPTERS. 



173 



CHAPTER XIX. 



SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS 

 ON HYBRIDISM. 



OX THE EFFECTS OF CROSSING — THE INFLUENCE OF DOMESTICATION ON FERTILITY 

 — CLOSE INTERBREEDING — GOOD AND EVIL RESULTS FROM CHANGED CONDITIONS 

 OF LIFE — VARIETIES WHEN CROSSED NOT INVARIABLY FERTILE — ON THE DIF- 

 FERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND VARIETIES — CONCLUSIONS 

 WITH RESPECT TO HYBRIDISM — LIGHT THROWN ON HYBRIDISM BY THE ILLEGITI- 

 MATE PROGENY OF DIMORPHIC AND TRIMORPHIC PLANTS — STERILITY OF CROSSED 

 SPECIES DUE TO DIFFERENCES CONFINED TO THE REPRODUCTIVE SYSTEM — NOT 

 ACCUMULATED THROUGH NATURAL SELECTION — REASONS WHY DOMESTIC VARIETIES 

 ARE NOT MUTUALLY STERILE — TOO MUCH STRESS HAS BEEN LAID ON THE 

 DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND CROSSED VARIETIES — 

 CONCLUSION. 



It was shown in the fifteenth chapter that when individuals 

 of the same variety, or even of a distinct variety, are allowed 

 freely to intercross, uniformity of character is ultimately ac- 

 quired. Some few characters, however, are incapable of fusion, 

 but these are unimportant, as they are almost always of a 

 semi-monstrous nature, and have suddenly appeared. Hence, 

 to preserve our domesticated breeds true, or to improve them 

 by methodical selection, it is obviously necessary that they 

 should be kept separate. Nevertheless, through unconscious 

 selection, a whole body of individuals may be slowly modified, 

 as we shall see in a future chapter, without separating them 

 into distinct lots. Domestic races have often been intentionally 

 modified by one or two crosses, made with some allied race, 

 and occasionally even by repeated crosses with very distinct 

 races ; but in almost all such cases, long-continued and careful 

 selection has been absolutely necessary, owing to the excessive 

 variability of the crossed offspring, due to the principle of rever- 

 sion. In a few instances, however, mongrels have retained a 

 uniform character from their first production. 



When two varieties are allowed to cross freely, and one is 



