14 



have been made but no serious investigation has taken place; no 

 attention was paid in the field to oecological factors and many 

 other important points; the investigations were often made in different 

 localities, so that an accurate comparison of the successive results 

 becomes impossible. Doubtless very valid excuses may be advanced 

 for the incompleteness of the investigations. But it is inexcusable 

 that on sucli grounds conclusions were based which could only be 

 justified if they had been founded on scrupulous researches. 



