Gr. H. F. Nutt all 
345 
adapted themselves to the altered conditions, finding in the fur or cloth-fibres 
constituting the clothing a substitute for the hair that was vanishing from 
the host’s body surface. 
Now, however, in view of the success that has been attained in modifying 
capitis in a short time so that it becomes indistinguishable from corporis, 
both structurally and biologically, I can but conclude that P. humanus is 
a species possessing considerable powers of adaptation, that the primitive 
form capitis, being in a labile state, can rapidly adapt itself to altered conditions 
beneath the clothing of man. Whilst in Sikora’s experiments, the change of 
character was effected in four to five generations, a longer period was required 
to produce the change in Bacot’s strains which we have studied. It is con¬ 
ceivable therefore that some races of capitis are more labile than others, but 
it is also likely that the change will be more rapidly effected by a graded 
process of feeding suited to bring about the rapid acquisition of the habit of 
gorging. There is little doubt in my mind that capitis is being converted into 
corporis to-day in nature, and that the latter, when man has become hairless, 
will constitute a species whose birth we are witnessing. 
Conclusion. 
In view of the morphological and biological evidence contained in the 
foregoing statement, incorporating all the information available up to the 
present, and reviewing the evidence critically in a manner that has not been 
done hitherto, I conclude that capitis and corporis merely represent two 
unstable races of one species 1 , Pediculus humanus Linnaeus, and that they 
should in future be designated as follows, partly from practical considerations: 
Pediculus humanus race capitis. 
Pediculus humanus race corporis. 
IV. 
PHTHIBUS PUBIS (Linnaeus 1758), Leach 1815. 
Synonymy. 
1034. Pediculus ferus Moffett 1634, p. 200; Ray, 1710, p. 8, who writes: “Numerose 
propagatur inter piles inguinum, praecipue immundorum,” they occur in the 
axillae, are hard to pull away, for they cling like Rieinus, and “raro item vagun- 
tur.”; Offers, 1810, p. 83. 
1071. Pediculus inguinalis Redi 1671, PI. XTX. 
— “ Pediculus morpio Merr. pin. 202,” cited by Linnaeus, 1761, p. 475. 
1 In my opinion, the two forms are insufficiently fixed to be regarded as varieties. In this 
connection it appears appropriate to quote the following passages from Poulton’s Essays on 
Evolution (1908): Darwin, writing to Asa Gray in 1850, states it as his opinion “that species are 
only strongly marked varieties.” In a letter to Hooker, Darwin wrote in 1804, “The power to 
remain for a good long time constant I look at as the essence of species, combined with an appreci¬ 
able amount of difference.” Coulton ( loc. cit. p. 70) writes that if two forms are widely different 
and still syngamic (interbreeding freely) they are to be regarded as one species (witness the various 
races of dogs, etc.). 
