STUDIES IN THK HYHIUD HISTONINAK. 

 TIL THE STIMULUS OK HETKROZYOOSLS. 



By J. W. MESLOP HARRISON, D.Sc. 



(With Two Text-figures.) 



Pahtly owing to the inriuence of Darwin and those who succeeded 

 him, and jwirily because it harmonises with the observed facts, the 

 dictum that cross fertilis^ition is a source of strength or of stimulus to 

 metabolic activity hjis become almost axiomatic ; very few workers, how- 

 ever, seem to have sjx'culated Jis to its method of action. 



Whether the phnise be restricted to the actual cross fertilisation 

 of plants or extended naturally, as ever3'one understands, to include 

 the act of avoiding inbreeding in animals, some explanation of the 

 phenomenon must exist, and it is now proposed to consider what light 

 hybridisation experiments in the Bistoninae throws on the matter. 



Almost immediately the experiments were initiated it was discovered 

 that the hybrid larvae were not only emphatically more robust than 

 those of the weaker parent, but they also surpassed in strength and 

 vitality those of the stronger form. For instance, attempts repeated 

 yeiir by year to rear Ni/.ssia gniecaria have uniformly failed ; yet when 

 this species was mated with Lycia Inrtaria the resulting larvae wt^-re so 

 sturdy and strong that, as larvae, their mortality rate was negligible 

 and, moreover, they fed up in the amazingly short period (»f six wcrks, 

 thereby anticipiting the pupation of the more vigorous parent by no 

 less than four weeks. Nor must this be deemed an isolated or exc('i)- 

 tional case; to a greater or less degree it illustrates the condition of all 

 the crosses. 



Since there were ditierences in these partieulars, as soon as the s»t of 

 experiments was completed, an endeavour was made to correlate the 

 degree of sturdincss and acceleration of development with other facts, 

 and this had striking success, 'leaking the ring <>! hybrids in which 

 Lycia figures as the central genus, not only because that circle is 

 practically complete, but also because the* crosses concerned in it had 



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