The bearing of the above observations on interspecific sterility.

These observations raise the question, to what extent, if any, an intersexual constitution may be resposible for the sterility observed in F. 1 hybrids in other interspecific pigeon crosses and in other species of birds.

Unfortunately the importance of intersexuality as a possible cause of sterility in these Stock-Dove Pigeon hybrids was not fully appreciated when the interbreeding was commenced in 1932.

The sex-organs of the chicks which died prematurely in the earlier years of the enquiry were not microscopically examined.

If the opportunity to repeat the same cross should occur to any one interested, it is much to be desired that accurate records should be kept of (1) the sex behaviour, (2) the fertility, and (3) the constitution of the sex-organs of the F. 1 hybrids in such a cross.

In the absence of absolutely conclusive evidence of the existence of testicular tissue in the ovaries of the hybrid birds now described we must regard the question of the influence of intersexuality on sterility as at present undecided, though I think that further observations will show that such an influence does exist.

In conclusion, I will mention two cases of crossbred, Br. Leg. × R.I. hens, which showed abnormal sex behaviour, and male secondary sex characters limited to the comb and wattles. In both cases the ovary contained a nodule of interstitial, but not spermatogenic testicular tissue.

(C. J. Bond, Journ. Genetics, vol. xxxvi. no. 3, with references to the literature on intersexual hybrids.)