6) 
38 Mr. J. W. Tutt on recent Experiments in 
whereas 7. bistortata is the more dominant of the two 
species J’. ab. delamerensis is dominant over typical 
T. crepuscularia. In cases in which ab. delamerensis is 
used as a parent, the number of individuals of, or approach- 
ing to, this form is rarely less than 50 per cent. In 
all cases where typical 7’. crepuscularia has been used its 
influence has been almost extinguished by that of 7. 
bistortata. This may probably be explained by the fact 
that at the present time 7’. ab. delamerensis is in rapid 
process of specialisation in its colour development, and 
that this activity is maintained in the crosses; or it may 
be explained as following the ordinary laws of natural 
selection, if it be conceded that variation is important to 
the existence of the species, since it is simply the survival 
of the fittest locally. This conclusion does not, then, 
necessarily conflict with the fact that the phylogenetically 
older form, which has survived as the fitter over large 
areas for a long period, is more potent}. 
8. The re-crossing of a hybrid with one of the parent 
species, produces offspring scarcely differing from the 
parent species with which the hybrid has been paired. 
[This conclusion is weak, so far as it is based only on a fairly 
large brood (49 specimens) bred by Mr. Bacot by pairing 
a dark @ hybrid (gf TZ. bistortata x T. ab. delamerensis) 
with a 7. crepuscularia (second brood). These are, 
therefore, three-fourths Z. crepuscularia and one-fourth 
T. bistortata ; the connexion of the latter species is only 
evident in a tendency to ochreous in the ground-colour of 
the males, although the latest emergences (7) of the 
females show very distinctly the 7. bistortata cross. With 
the exception of the slight change in ground-colour, all 
the other specimens are practically indistinguishable from 
typical 7. crepuscularia.] 
9. The inbreeding of the hybrids of the same cross 
with each other produces a large percentage of individuals 
differing much from either parent form. [The form 
frequently produced by this cross has already been 
described and compared with female Z. consonaria, which 
it superficially resembles in some respects. The same 
form, but of a rather less pronounced type, occurs 
occasionally in first crosses.] 
10. The crossing of the hybrids obtained from original 
reciprocal crosses tends to produce a mixed progeny, some 
referable to known forms of the crossed species, others 
