( cxiiv ) 



hosts better than do the hosts themselves, just as certain 

 caterpillars have indicated a relationship of their food-plants 

 once considered to belong to widely different orders. 



It must not, however, be concluded that the species 

 of Epizoa are constant. They too are liable to variation, 

 especially those which, like the fleas, do not spend all their 

 life on the host. We find among them many instances of 

 conspicuous variation, individual as well as geographical. 

 The flea of the hedgehog, for instance, is different in the 

 western Mediterranean countries from the form found in 

 Central Europe and Great Britain. The rodent flea, Ctenoph- 

 thalmus agyrtes (very common in Great Britain on voles 

 and mice), has developed into a number of geographical races 

 on the Continent, and even the Scottish and British specimens, 

 taken as a whole, show some distinctions. Text-figs. 1-5 

 represent a portion of male genital organs of five fleas repre- 

 senting Ctenophthalmus agyrtes, in various parts of Europe : 

 agyrtoides Wahlgr. (1911) from Scandinavia, eurous Jord. 

 and Roths. (1912) from Hungary and Russia, agyrtes Hiller 





Figs. 1-5. Portion of $ -genitalia of Ctenophthalmus agyrtes and its 

 geographical representatives: 1. agyrtoides; 2. agyrtes; 

 3. eurous; 4. provincialis; 5. baeticus. 



(1896) from Central Europe, Northern France and the British 

 Isles, 'provincialis Roths. (1910) from the French Alps and 

 Southern France, and baeticus Roths. (1910) from Portugal. 

 In America, North of Mexico, we know a number of species 

 which also consist of four or five geographical varieties, and 

 the same can be said of certain species inhabiting other 

 continents. So far as our knowledge goes at present, this 

 geographical variation of the fleas is not dependent on differ- 



