THE THEORY OF SEX 597 



stage is finally reached. The next organ to become involved is 

 the copulatory organ — details will be discussed later — which 

 starts intersexuality with the animal in figure 6 and reaches 

 practically complete maleness with the end of the series. Then 

 follows the number of ripe eggs in the abdomen, decreasing from 

 some hundreds to a few — the individuals in figures 11 to 15. 

 Then follow color and hair of the abdomen, beginning to be more 

 or less male with the animal in figure 11. Then follows the shape 

 of the abdomen which is practically male with the animal in 

 figure 15. Only then does the size of the abdomen begia to 

 change, finally becoming male. But the sex-gland is — apart 

 from details, which will be mentioned later — still female. And 

 only in animals of the type of the last one (fig. 19) is the gradual 

 transition of the ovary into the testis observed. 



Very similar is the series of male intersexuality (figs. 20 to 

 33), which explains itself. It is to be noted that the female 

 type of abdomen begins with the animal in figure 28. The last 

 of the series (fig. 33) had a practically female abdomen. And 

 it might be added that the change of the instincts begins in the 

 female series with the animals in figures 9 to 11, the four last 

 ones exhibiting practically male instincts. In the male series 

 the change begins with the animal in figure 31, only the last of 

 the series behaving practically like a female. 



If we now try to formulate a rule which governs this strange 

 seriation, as well as the seriation in the finer details of certain or- 

 gans, we find the most important fact, thai this series is the inverse 

 of the order of differentiation of these organs in development. The 

 last organs to differentiate in the pupa and the first to be inter- 

 sexual are the branching of the antennae and the coloration 

 of the wings. The first imaginal organ differentiated in the 

 caterpillar and the last in the series to be changed toward the 

 other sex is the sex-gland. And if we apply this law even to the 

 minute parts of a single organ like the copulatory organ, we 

 find it also to apply, as will be demonstrated later. Now this is 

 the fact which, in connection with the others, enables us to 

 formulate a definite physiological theory of sex-determination, 

 which we propose first to consider and later to test with the 

 :acts. 



