36 Mr. J. W. Tutt on recent Experiments in 
ings, and weak colour. It has been deemed advisable, 
however, to classify these specimens as ab. delamerensis, 
The conclusions to which these experiments point may 
be briefly summarised as follows :— 
1. The intercrossing of the two species may result in 
every possible intermediate stage of fertility, from complete 
sterility to the production of the full number of fertilised 
eggs. These extreme results may even happen when 
dealmg with different individuals of the same _ brood. 
[Failure, therefore, in a few individual cases must not be 
taken as proving that any particular cross is infertile. 
Individual aberration of the genital organs may lead to 
failure; one of Dr. Riding’s failures happened with a pair that 
remained in copuld two days and then had to be forcibly 
separated ; this probably was due to such an aberration.] 
2. Certaim crossings produced almost entirely male 
offspring. [These were (a) the cross 7. ab. delamerensis 
g xT. bistortata 9. Oftwo broods of this crossing, the 
larvee of which were mixed, Dr. Riding obtained 60 males 
and but 1 ill-developed female. A cross of the same 
two forms, as male and female parent respectively, by 
Mr. Bacot produced 58 males and no female. It would 
have been easy here, had Dr. Riding’s one ill-developed 
female not emerged, to have assumed that this cross 
would produce only male progeny. (b). The cross in which 
T. crepuscularia gf was paired with 7. bistortata 2 
produced in Dr. Riding’s hands 38 males and not a single 
female. It is worthy of note that these were the only 
four fertile crosses obtained in which 7. crepuscularia was 
the male parent. | 
3. The hybrids are fertile enter se but to a less extent 
than in the parent stock, 7.e¢., there appears to be a larger 
proportion of failures. They are also fertile with the 
parent stock. [The former part of this conclusion 
has been amply proved by both Dr. Riding and Mr. 
Bacot. The latter was proved by Mr. Bacot successfully 
crossing a 2 TZ. bistortata x delamerensis with a g T. 
crepuscularia. One might have expected that if the males 
of a hybrid were capable of fertile union with females of 
either parent species as shown by Standfuss, that a certain 
percentage of the females should be so, and probably also 
with each other. The number of specimens at the 
disposal of Dr. Riding and Mr. Bacot has made their 
experiments particularly useful, as it allowed for a large 
