476 LÉPIDOPTEROLOGIE COMPARÉE 



the female sex occurs, and that of two forms. Considering the 

 case of hybr. denhami {hïrtaria cf x zonarïa Q ) ; whilst it is 

 true that this is the cross which occasionally gives infertile ova, 

 i. e. ova which do not develop at ail, direct experiment shows that, 

 under favourable conditions, ail the ova from single females 

 hatch and yet no females are produced. 



Secondly, if the factor /. zonaria be replaced by /. grœcaria, — 

 a species closely allied to zonaria, as has been shown elsewhere, 

 but which has ova indistinguishable from those of hirtaria, ■ — 

 again only maies are obtained, and, in the two crosses actually 

 made, every ç:g^ hatched. 



Incidentally, that hirtaria cf is not primarily responsible for 

 thèse phenomena can be seen from crossings 2 and 3, in which 

 it takes no part; and yet, under ordinary circumstances, they yield 

 no females. 



It may then be advanced that inability of the embryonic 

 changes (due to the présence of two species) to synchronise, owing 

 to the différence in steps necessary to produce two females of such 

 diverse forms as zonaria Q and hirtaria Q, may be the cause. 

 This is, at first sight, confirmed by the observation that this cross 

 very often produces ova in which the larva develops to a certain 

 extent and then f ails to hatch, but is disproved by the same séries 

 of experiments as is detailed above ; and f urther by the f act that 

 a Crossing of normal pomonaria cf with a similar zonaria Q, gives 

 no females, in spite of the fact that the structural différences of 

 zonaria Q and -pomonaria Q are but small. 



Again, no such interférence occurs in the case of ail the reci- 

 procal crosses or in the Lycia-Pœcilopsis hybrids. 



Next, we come to théories based on the possible cytological 

 différences in the ova and the spermatozoa. 



Thèse can be resolved into two suppositions : 



I. We can assume a différence in the size between the sperma- 

 tozoa destined to produce maies and those destined to give 

 females. 



