2o 4 THE VARIATION OF ANIMALS IN NATURE 



Gardens. The snails were put into the enclosures in which 

 the pheasants were kept in such a way that all four types were 

 accessible to the birds. 



There were six experiments with six different species of 

 birds (including one hybrid) . 



In experiments I— III and VI (Phasianus colchicus colchicus, 

 P. c. torquatus, Crossoptilon mantchuricum and Lophophorus impejanus) 

 no selection of any type was observed. In experiment IV 

 (hybrid of Chrysolophus pictus and amherstiae) the dark shells were 

 taken and the light and banded left. The author does not 

 state what background these forms were on, except to say that 

 the dark forms were difficult for the human observer to see, and 

 that he thought the birds revolted from the light-coloured 

 snails. In experiment V (Gennaeus nyctimerus) the dark forms 

 and red and yellow unbanded forms were taken and the banded 

 left alone. He says that on the pale greenish-yellow grass in 

 the enclosure the banded snails were inconspicuous to the 

 human eye. 



The value of these experiments is very problematical. 

 The author admits that the captive birds are accustomed to 

 being fed by the public. He does not mention how many 

 experimental snails were used. In the two cases in which he 

 claims that selection of certain types was observed, he says 

 (experiment IV) one kind was taken ' zuerst ausnahmlos ' ; 

 in his second, that the selected types were ' grossenteils 

 gefressen.' 



(10) Lutz {191 5) : experimental observations on Drosophila. 



This author studied the effect of starvation on D. ampelophila 

 in relation to the duration of the embryonic period and on 

 two structural characters (length of first posterior cell in wing 

 and breadth of wing). 



Two methods were adopted : (i) the comparison of the 

 mean of the characters of the survivors and eliminated ; and 

 (ii) the correlation of a given character and the ability to 

 survive. 



(a) There was a negative correlation between the length 

 of adult life and the duration of the embryonic 

 period. Those with shortest embryonic period lived 

 the longest. 



