As evidence in favour of his view, he points out that these excrescences are, in many species, not only functionless but absolutely injurious, as in the case of the comb and wattles of the jungle cock and his domestic descendants, which merely serve as a handle for enemies to seize. Cunningham asserts that the only objection to his theory is the dogma that acquired characters cannot be inherited. This assertion is, however, not correct. It is, indeed, a very serious objection that all the evidence available seems to show that acquired characters are not inherited, but this is by no means the only difficulty. Before mentioning these further objections, let us say a word on the subject of the inheritance of acquired characters. Mr Cunningham himself compares the formation of a splint or spavin in a horse as the result of special strain, to the acquisition of secondary sexual characters. Unfortunately for Cunningham’s theory, but fortunately for mankind in general, spavined horses and mares do not beget spavined offspring. If, then, spavin is not inherited, is it not unreasonable to assert that the thickening of the bone that develops on the head of a butting animal is inherited? Another objection to Cunningham’s theory is that many birds which show off their plumage most vigorously possess no ornamental plumes. As Howard has recorded, many of our dull-coloured British warblers show off in the same manner as bright-coloured birds do. If the exercise has caused the development and inheritance of plumes in some species, why not in the others? Again, Cunningham is not correct in saying that sexual dimorphism is “virtually absent” in the Columbidæ. Few birds display so striking a sexual dimorphism as the Orange Dove (_Chrysœna victor_) of Fiji, in which the male is bright orange and the hen green. We have already cited the case of the curious sexually dimorphic red turtle-dove. Now, the courting attitudes and actions of this species are precisely the same as those of other allied turtle-doves; why, then, have these exercises caused only one species to become sexually dimorphic? Existing Theories not Satisfactory Our survey of the more important attempts which have been made to explain the phenomena of sexual dimorphism leads to the conclusion that these still require elucidation. We have weighed each theory in the balance and found it wanting. The outstanding feature of sexual dissimilarity is the apparently haphazard manner of its occurrence.