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 INVARIABLY FERTILE--ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND VARIETIES--CONCLUSIONS WITH RESPECT TO HYBRIDISM--LIGHT THROWN ON HYBRIDISM BY THE ILLEGITIMATE 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 ... Page 173