There is a race of fowls known as Campines in which there are two kinds of males, hen-feathered males and cock-feathered males. If the hen-feathered male is castrated, the new feathers that develop are the long feathers of the cock-feathered male, Fig. 10. In another race of fowls, Sebright bantams, only the hen-feathered males are known. If these are castrated, the new feathers that develop are the long feathers characteristic of all other races of poultry, Fig. 11. [Illustration: FIG. 11. To left (a) hen-feathered Sebright cock. To right (b) castrated Sebright cock that has developed characteristic cock-feathering.] If the Sebright male is out-crossed to a hen of another breed in which only cock-feathered males occur, it will be found that all the first generation males are hen-feathered. If these are now bred to their sisters there are produced, in the second generation, three hen-feathered males to one cock-feathered male, showing that the difference between the two races is inherited, Fig. 12. [Illustration: FIG. 12. Cross between hen-feathered Sebright cock and black-breasted game female belonging to a race with cock-feathered males. The offspring (F₁) are hen-feathered males and normal hens. These inbred give 3 hen-feathered to 1 cock-feathered son.] Now in this case we can perhaps go further. An examination of sections of the testes has shown that in the hen-feathered Sebright male there are certain kinds of cells, called luteal cells, while these are absent in the sections of the testes of normal cocks. These same luteal cells are like those present in the stroma of the ovary of all female birds. If we assume that they make an internal secretion that prevents the development of cock-feathering, both in the normal hen and in hen-feathered cocks, we have a complete explanation of all the facts. This explanation is made more probable by the results of removing the ovary of the hen, when, as Goodale has shown, the spayed hen develops the full male plumage of her breed. Since the luteal cells are present in the hen and in the hen-feathered cock, and are absent in the adult cock-feathered male, it seems not a far-fetched hypothesis to assume that these cells (or their secretions) are those involved. The next illustration carries its into a more debatable field. Many human defects are connected with the nervous system, and it is interesting to find that many of them are believed to be inherited; even when no corresponding structural basis in the brain can be made responsible for the defect.