﻿HYBRIDS OF THE GENUS OPORABIA. 31 



origin of some species. Suppose a mutation of a species to arise 

 through some disturbance in the normal behaviour of the chromo- 

 somes ; let this mutation cross with the parent form ; imagine 

 the difference in rate of metabolic change to result in precocious 

 development of the resulting imagines. There would then 

 result a number of insects emerging some time before the 

 parent, and not necessarily sterile. They would, however, be 

 effectually separated from the original form by a certain space 

 of time. They would thus be saved from being swamped, and 

 differing, as was granted, in the chromosomes, they must in the 

 process of time be regarded as a " new species." If such be the 

 origin of some species, we ought to be able to find pairs of closely 

 allied species separated from each other by the time of emergence. 

 Such pairs we see in Biston strataria and Amphidasis hetularia, 

 and Tephrosia [Ectropis) bistortata and T. crepuscularia. 



Let me now take up point two. 



That the accelerated form resembles a melanic dilutata may 

 be a consequence of that acceleration. It may be possible that, 

 in dilutata, when once the imago commences to develop, it forms 

 at a much more rapid rate than in autumnata, although the 

 periods of the commencement of development may not coincide, 

 for, as a matter of fact, autumnata commences to develop first. 

 The acceleration produced otherwise in the females may cause 

 the dilutata-ness of the hybrid to overpower to some extent its 

 autamnata-neas. However, it is not, I think, to this we have to look 

 for our explanation, but to the constitution of the parent gametes. 



Experiment has shown that, as regards sex, in the Lepidoptera 

 one of the two sexes is heterozygous.* The bulk of the evidence 

 shows that it is the female that is heterozygous and the male 

 homozygous. In Satwrnia, however, I have definite evidence 

 that it is the male that is heterozygous, whilst, in the Bistoninse, 

 the weight of the evidence is in favour of the view that the 

 female is heterozygous. 



Suppose in the genus Oporahia that the male is heterozygous 

 and has the composition ^ $ ,f whilst the female is homozygous 

 and is represented as ? ? . Imagine, too, that the form of 

 dilutata used is melanic, and therefore carries a factor for 

 melanism. Let us further suppose that, in the gametogenesis, 

 there is a kind of false allelomorphism between maleuess and 

 the melanic factor, and that they repel each other. Spermatozoa 

 would thus be formed of two kinds, one carrying femaleness and 

 melanism ( ? M),t and the other maleness and no melanic 

 factor ( ^ N).I The females, on the other hand, would give ova 



--■■ Castle (Bull. Mus. Comp. Zool, Harvard, 1903, vol. xl. No. 4) says 

 that possibly both sexes are heterozygous in some forms in this respect. 



f Maleness being regarded as dominant. 



I M is used to represent the presence of a factor for melanism ; N de- 

 notes its absence. 



