K. Jordan and N. C. Rothschild 
9 
Pulex simulans Baker (1895, pp. 65 and 67, Texas, off Didelphys virginiana)', Webster 
(1904, p. 244). 
Pulex irritant, var. dugesi Baker (1899, p. .37, Mexico, off Spermophilus). 
Pulex irritant var. simulant Baker (1904, pp. 379 and 457, occurrence on Didelphys 
accidental). 
Pulex dugesi Baker (1904, p. 379); Baker (1905, p. 129). 
Linne recognised two species of fleas only, Pulex irritans, which he 
diagnosed as P. proboscide corpore breviori, in contradistinction to the 
“ chigoe,” Pulex penetrans, of which he says P. proboscide corporis 
longitudine. Linne’s P. irritans comprised various very distinct species 
of flea, such as those found on the rabbit and dog, in addition to the 
human flea, the true P. irritans. Leach, Bouche, Taschenberg and 
other authors have properly restricted the name irritans to the flea 
parasitic on man, the names vulgaris, atev and hominis of Raius, Linne 
and Duges being treated as synonyms. The nearest allies of this 
species which are found on various animals are all inhabitants of the 
Old World. The fleas found in America are only distantly related 
to P. irritans. It may therefore be concluded that the present species 
originated from the Old World stock of Siphonaptera. Our knowledge 
of the variation and the present and former distribution of this insect 
is extremely meagre. Nevertheless, there are some points to which 
attention may with advantage be drawn. The insect is practically 
cosmopolitan, or, rather, has become cosmopolitan; specimens identical 
with European irritans are found almost everywhere. This was 
probably not the case prior to the introduction of the systems of 
intercommunication between all parts of the globe such as have been 
created in more recent periods. The tropical countries of the eastern 
hemisphere do not appear to be inhabited by P. irritans except where 
European settlements exist, or, at least, where intercourse with 
Europeans goes on. Ports, apparently, are everywhere infested by this 
parasite. We have failed to find any definite records of fleas in books 
on travel published in the early part of the nineteenth century. 
Although travellers in the tropics of Africa and Asia frequently 
complain of the abundance of lice on the natives in their huts, yet fleas 
do not seem to have been observed. 
If it is a fact, however, that the tropical countries of the Old World 
were originally devoid of Pulex irritans, the reasons for the absence of 
the insect in question are worth considering. Two causes, of a bio¬ 
logical character, may conduce to the absence of a species from any 
particular district. On the one hand the conditions of life may not 
