46 



vi. Isolatio7i or Segregatioji. 



(Lloyd Morgan, Animal Life and Intelligence, pp. 99-112; 

 Romanes, Physiological Selection, Journ. Linn. Soc, Zool., 

 Vol. 19, pp. 337-411, 1886; GULICK, Divergent Evolution 

 through Cumulative Segregation, Journ. Linn. Soc, Zool., 

 Vol. 20, pp. 189-274.) 



The difficulties of Natural Selection viewed as the 

 sole cause of evolution. 



(i) The difference between natural species and 

 domesticated varieties in respect of fer- 

 tility when crossed. 



(2) General inutility of specific characters. 



(3) Swamping effects of intercrossing. 



Importance of Segregation, or the prevention of in- 

 tercrossing, in the origin of domesticated varieties. 



Modes of Segregation in nature. 



Geographical. 



Variations of habits. 



Preferential mating. 



Particulate inheritance. 



Physiological isolation. 

 Evidences for physiological isolation. 



Immense number of variations. 



Sexual organs most variable. 



Variation often toward sterility. 



