INFESTING THE DIFFERENT RACES OF MAN. 569 
consequence ; but it is not wholly so. The animal owes its colour to two differ- 
ent sources: the one (its true colour) to its chitonous external coveringe—this 
colour it cannot lose; the other (its accidental colour) to the food it has swallowed. 
Being semi-transparent, the contents of the body are seen through the chitonous 
skin or outer skeleton. This colour it loses when the intestines, &c., are emptied. 
I have seen a living specimen of the dirty-coloured European Pediculus, after a 
few days’ abstinence in a quill, lose all its dirty colour, and become entirely of a 
pale horny, almost colourless, hue, which is the natural colour of its external 
skeleton. The black Pediculus, gorged with the blood of a negro, would, I have 
no doubt, under similar treatment, get rid of the blackness due to the contents of 
the body, but retain that amount of brown tint which belongs to the external 
covering. Colour, therefore, is not to be wholly discarded ; and when it goes 
along with other distinctions, may be properly used as a subsidiary, although 
subordinate character in determining species. 
A difficulty of a different kind arises from there being, or being supposed to 
be, more than one species infesting the same race or the same individual. There 
are usually said to be four species found on Europeans. One of these (Phthirius 
inguinalis) is, however, not a true Pediculus, and therefore does not fall within the 
limits which I have assigned to myself in this paper—limits which I have adopted 
not from any wish or intention so to limit myself, but solely from necessity, as I 
have not been fortunate enough to procure any specimens of Phthirius from other 
nations. That other varieties or species of Phthirius exist, however, is possible, for - 
we hear of a parasite specially infesting the eye-lashes of some of the natives in the 
East Indies, which probably may be a Phthirius. Butas I have not received any 
specimens of this or any other exotic Phthirius, Iam compelled to leave them out 
of the inquiry. This does not, however, at all interfere with the accuracy of the 
results drawn from the Pediculi proper, as the Phthirius is generically distinct from 
the Pediculus, the grasping or scansorial claws, as they are called, being on the 
posterior feet instead of the anterior, so that they cannot possibly be confounded 
with them. A third so-called species of Pediculus may also be discarded from our 
consideration, as having no existence in nature—the P. tabescentium, said to be 
peculiar to diseased persons. Denny, in his work on the Anopleurze, seems to 
believe in it; but he never saw it, doubtless for the best of all possible reasons, 
viz., that it does not exist. Every instance of the supposed occurrence of this 
species, which has fallen under the notice of any competent observer, has turned 
out to be merely an unusual profusion of the common Pediculus vestimenti. In 
the only instance which fell under my own observation, this was the case. Dr 
Jackson of this city, some years ago consulted me regarding a case of the 
kind, where a patient, a young lad, was supposed to be infected. Repeated 
washings seemed to have no effect upon this terrible disease. A few hours after 
washing, the unfortunate victim was found again to be swarming as badly as 
