Chap. XIX. SUMMARY OF THE FOUR LAST CHAPTERS. 157 



CHAPTER XIX. 



SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS ON 



HYBRIDISM. 



ON 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 IN- 

 VARIABLY FERTILE — ON THE DIFFERENCE IN FERTILITY BETWEEN 

 CROSSED SI'ECIES AND VARIETIES — CONCLUSIONS WITH RESPECT TO 

 HYBRIDISM — LIGHT THROWN ON HYBRIDISM BY THE ILLEGITIMATE 

 PROGENY OF HETEROSTYLED 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 FEKTILITY 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 often of a 

 semi-mon stron s 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, a whole body of 

 individuals may be slowly modified, through unconscious 

 selection, as we shall see in a future chapter, without separa- 

 ting 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 cros>es with 

 very distinct races ; but in almost all such cases, long-con- 

 tinued and careful selection has been absolutely necessary, 

 owing to the excessive variability of the crossed offspring, 

 due to the principle of reversion. In a few instances, how- 

 ever, mongrels have retained a uniform character from their 

 first production. 



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



