Iv. Jordan and N. C. Rothschild 
11 
The division of mankind into various races, many of them as distinct as 
the various species of some genus among other mammals, would lead 
one to expect that a corresponding differentiation would have taken 
place among the fleas parasitic on them; and that the human flea 
would now consist of a number of different races each peculiar to its 
particular human host. Some development of this nature in fact 
appears to obtain in at least one instance. Dr Carl Baker (1899, p. 37) 
described a flea found by Dr Duges in West Mexico (Guanajuato) on 
Citellus macrourus. This insect Dr Baker considered at first to be 
a variety of Pulex irritans, calling it Pulex irritans var. dugesi, but 
later he treated it as a distinct species. We have two examples of this 
dugesi, for which we are indebted to Dr Baker, and possess also both 
sexes of the same form taken off Mexican Indians at Tabasco, which we 
received from the late Dr Buller. The differences between these 
Mexican dugesi and the true Pulex irritans are slight, but, nevertheless, 
fairly constant. The specimens are smaller in size, the rostrum (labial 
palpi) is longer, and the large flap of the clasper is less rounded at 
the apex. From an examination of our series of Pulex irritans from 
various South American countries, we find that a number of $ examples 
found on Gonepatus in Bolivia agree with dugesi in respect to the 
length of the rostrum, but are not inferior in size to the true irritans. 
Other examples from Peru, Chile, Argentina, Paraguay and Brazil, 
taken in houses and on various mammals, do not differ from typical 
European irritans. The occurrence of a slightly different race of 
Pulex irritans in Mexico and Bolivia, and possibly in the inter¬ 
mediate countries, both on the natives and on mammals, raises two 
interesting points, first, whether Pulex irritans originated as a flea 
found on mammals and then adopted man as its host, or if it developed 
to what it now is on becoming a parasite of man. In Europe and 
Central Asia this flea is essentially a parasite of man, occurring only 
occasionally on other warm-blooded creatures. The same may be the 
case in other countries, the frequent occurrence of this insect on 
mammals in America being possibly explained by the closer connection 
between human dwellings and wild and semi-domesticated beasts. 
Dirty and deserted huts frequented by small mammals may also induce 
the human flea to propagate more freely. As we have pointed out the 
imagines of the human flea can exist a long time without food, while the 
larvae, as opposed to being parasitic, feed on all kinds of dirt. In 
respect to the second point mentioned above, namely, whether the 
present species has undergone modifications since becoming a parasite 
