Richard Goldschmidt has discovered in his breeding experiments on the gipsy-moth (Lymantria dispar) a phenomenon which will probably throw much light on the physiology of sex determination. He found that certain crosses between the Japanese and the European gipsy-moth do not give pure sexes, males or females, but mixtures of the sexual characters of both sexes, and this mixture is a very definite one for definite crosses. These differences are such that it is possible to grade the hybrids according to their manifestations of maleness or femaleness, both in morphological characters and instincts. Goldschmidt calls this peculiar phenomenon intersexualism, and its essential feature is that the various degrees of intersexualism can be produced at will by the right combination of races.
Female intersexualism begins with animals which show feathered antennæ of medium size (feathered antennæ are a male character), but which are otherwise entirely female in appearance except that they produce a smaller number of eggs which are fertilized normally. In the next stage patches of the brown male pigment appear on the white female wings in steadily increasing quantity. The instincts are still female, the males are attracted and copulate. But the characteristic egg sponge laid by the animal contains nothing but anal hairs in spite of the fact that the abdomen is filled with ripe eggs. In the next stage whole sections of the wings show male colouration, with cuneiform female sectors between, the abdomen becomes smaller, contains fewer ripe eggs, the instincts are only slightly female, the males are attracted very little, and reproduction is impossible. In the next stage the male pigment covers practically the whole wing, the abdomen is almost male, but still contains ovaries with a few ripe eggs, the instincts are intermediate between male and female. Then follow very male-like animals which still show in different organs their female origin and have rudimentary ovaries. . . . The end of the series is formed by males, which show in some minor characters, such as the shape of wings, still some traces of their female origin.
The series of the male intersexes starts with males showing a few white female spots on their wings. These become larger and larger, the amount of brown pigment correspondingly decreasing. . . . Hand in hand with this the abdomen increases in size, reaching in the most extreme cases two-thirds of the female size (without containing eggs). The same is true for the instincts which become more and more female.
(And also for the copulatory organs which also become more and more female.)
As stated above, the main fact that every desired degree of intersexualism can be produced at will by properly combining the races for breeding, and the intersexual potencies of the different races has been worked out by Goldschmidt.[195]
6. The relation between chemical substances circulating in the body—either derivatives of food taken up from without or of chemical compounds formed naturally inside the body—and the production of sexual characters is best shown in the polymorphism found among the social ants, bees, and wasps. Here we have, as a rule, in addition to the two sexes a third one, the workers, which are in reality rudimentary and for that reason sterile females. They differ more or less markedly from both the typical male and female in their external form, and, as a rule cannot copulate owing to their deficient structure. This third sex, the sterile neuters, can be transformed at desire into sexual females in certain species, as P. Marchal has demonstrated. He worked with a form of social wasps in which the workers are sterile and smaller than the real females. In such a society of wasps all the males and workers die in the fall and only the fertilized females survive, each one founding a new nest in the following spring. From the first eggs laid, workers arise, small in stature and sterile; these workers are nourished by their mother. Then these workers take care of the feeding of all those larvæ which arise from the eggs which their mother continues to lay. Throughout the spring only workers arise from the eggs. The males appear in the summer, the real females towards the end of the season when the sexes copulate.
Marchal isolated a number of the sterile workers, providing them with food but giving them no larvæ to raise. He found that the workers which thus far had been sterile became fertile, producing, however, only males. This latter fact is easily understood from what has been said regarding the bees, namely, that the female produces only one type of eggs, hence the unfertilized egg can give rise only to males. The astonishing or important point is that the ovaries of the workers begin to develop as soon as they no longer have a chance to nourish the larvæ, provided the food which would have been given to the larvæ is now at their disposal. In other words, the development of their ovaries is the outcome of eating the food which under normal conditions they would have given to the larvæ. The food must, therefore, contain a substance which induces the development of eggs. The natural sterility of the neuters or workers is, therefore, to use P. Marchal’s expression, a case of “food castration,” (“castration nutriciale”).[196] The workers originate from fertilized eggs and are therefore females, but for the full development of the ovaries and the other sexual characters something else besides the XX chromosomes is needed and this is supplied in this case by the quantity or quality of the food. May we not conclude that the same thing may happen generally, except that these substances are formed by the body under the normal conditions of nutrition through the influence of constituents of the second X chromosome?
It is known that the future queens among the bees receive also a special type of food which the workers do not receive. Again the idea of “food castration” of the latter is suggested.
In rotifers Whitney[197] has shown that the cycle in the production of males and females can be regulated by the food. In some species a scanty supply of green flagellates produced purely female offspring, while a copious diet of the same green flagellates produced a predominance of male grandchildren, sometimes as high as ninety-five per cent. This was confirmed by Shull and Ladoff.[198]