HYBRIDISATION 297 



were extensively infested, no transfer to apple took place. For 

 further information and references to the principal literature on 

 Plesiocoris vide Fryer (1929). 



Hybridisation Experiments. Evidence of the nature of the 

 food-preferences of crosses between biological races and sub- 

 species is of a very scanty nature. In this connection the 

 experiments of Gdschen (1913), therefore, are of particular 

 interest. The European Sphingid moth Celerio euphorbice is 

 represented by two races or sub-species named euphorbice and 

 mauritanica respectively. In both races the usual food-plant is 

 Euphorbia and neither will accept Salix, Goschen found that the 

 hybrid mauritanica S X euphorbice $ and known as wagneri, and 

 also the reciprocal cross, both can be reared to maturity without 

 difficulty upon Salix. Similarly the hybrid known as C 

 kinderwateri, produced by the cross euphorbice S X galii $, feeds 

 readily upon Salix, a food-plant upon which neither of the parents 

 naturally occur. On the other hand, observations made by Field 

 (1910), in North America, appear to show that host-selection is not 

 necessarily affected by hybridisation. Field's observations bear 

 upon the feeding propensities of the butterflies, Basilarcha arthemis, 

 B. astyanax and B. proserpina, the last named being regarded as a 

 natural hybrid between the first two. Larvae produced from a 

 captured proserpina female refused birch, sallow and poplar, the 

 usual food-plants of arthemis, and were reared upon wild cherry, 

 which is one of the favoured plant-hosts of astyanax. These 

 same larvse finally produced all the three forms of Basilarcha 

 named. He suggests that the constitution of the original 

 parent was proserpina (hybrid) x arthemis (colour recessive), 

 since astyanax does not prevail in the district where the insect 

 was captured. The unexpected preference of the astyanax food- 

 plant is explained as having been inherited from a grandparent 

 of that form. Unfortunately, neither Goschen nor Field were 

 able to continue their observations to later generations of the 

 species mentioned. 



The Interpretation of Host-selection. It has been seen that 

 the results of recent research afford definite support in favour 

 of Hopkins' host-selection principle in a considerable proportion 



