It does not seem to me correct to say that they serve to drive the flies away, because they are well developed in reptiles and fish, and in many animals of which the skin is insensible to the stings of insects; also, if the fly-hypothesis were the correct one, the cutaneous muscles should be best developed on those parts of the animal which cannot easily be reached with the head, the leg, or the tail, but the contrary is the case. Certainly the muscles are made use of for this purpose, but this is an accidental fact, as is also, I believe, the circumstance that these muscles serve to erect the hair when the animal is excited or afraid. When one dog approaches another in a hostile mood, there is such an agitation of his nervous system that he begins to tremble, not through fear, but excessive excitement. All the muscles contract, those of the blood-vessels, of the bladder, of the intestines, therefore it is comprehensible that the cutaneous muscles should also contract, raising the hair on the dog’s back. If we look at the skin of the arms or legs when we step into a cold bath, or when we uncover ourselves on rising in the morning while the temperature of the room is low, we notice the appearance of goose-skin. Whenever there is, for some reason, a contraction of the blood-vessels, these muscles contract also and the hair rises. The simultaneous appearance of these two phenomena is, I believe, useful to the animal, because, in raising the hair or feathers, the stratum of air enclosed by these appendages is increased, the loss of heat in this way diminished, and the cooling of the skin prevented. It is, perhaps, for this reason that horses, dogs, cats, and birds ruffle their hair or feathers when cold.[27] [27] Darwin gives another explanation of this phenomenon which seems to me less probable. He states that animals erect their dermal appendages that they may appear larger and more terrible to their enemies.