The question which of the two sexes contains the unpaired factor is less easy to answer, but there are several converging lines of evidence which point to the deduction that in Vertebrates at least, and in some other types, it is the female, and I feel little doubt that we shall succeed in proving that in them femaleness is a definite Mendelian factor absent from the male and following the ordinary Mendelian rules. Before showing you how the Sweet Pea phenomenon aids in this inquiry I must tell you of some other experimental results. The first concerns the common currant moth, _Abraxas grossulariata_. It has a definite pale variety called _lacticolor_. With these two forms Doncaster has made a remarkable series of experiments. When he began, _lacticolor_ was only known as a female form. This was crossed with the _grossulariata_ male and gave _grossulariata_ only, showing that the male was pure to type. The hybrids bred together gave _grossulariata_ males and females and _lacticolor_ females only. But the hybrid males bred to _lacticolor_ females produced all four combinations, _grossulariata_ males and females, and _lacticolor_ males and females. When the _lacticolor_ males were bred to _grossulariata_ females, whether hybrid, or wild from a district where _lacticolor_ does not exist, the result was that all the males were _grossulariata_ and all the females _lacticolor_! It is difficult to follow the course of such an experiment on once hearing and all I ask you to remember is first that there is a series of matings giving very curious distributions of the characters of type and variety among the two sexes. And then, what is perhaps the most singular fact of all, that the wild typical _grossulariata_ female can when crossed with the _lacticolor_ male produce all females _lacticolor_. This last fact can, we know, mean only one thing, namely that these wild females are in reality hybrids of _lacticolor_; though since the males are pure _grossulariata_, that fact would in the natural course of things never be revealed. When we encounter such a series of phenomena as this, our business is to find a means of symbolical expression which will represent all the factors involved, and show how each behaves in descent. Such a system or scheme we have at length discovered, and I incline to think that it must be the true one. If you study this case you will find that there are nine distinct kinds of matings that can be made between the variety, the type and the hybrid, and the scheme fits the whole group of results. It is based on two suppositions: 1. That the female is cross-bred, or as we call it heterozygous for femaleness-factor, the male being without that factor. The eggs are thus each destined from the first to become either males or females, but as regards sex the spermatozoa are alike in being non-female. 2. That there is a repulsion between the femaleness-factor and the _grossulariata_ factor.