INSECTS AND OTHER ORGANISMS 407 



hundred different passerine birds of various families — • 

 larks, finches, thrushes, and others — inhabiting both 

 Eurasia and America. V. L. Kellogg, in a highly interesting 

 discussion (1913) of the Mallophaga and their hosts, points 

 out that such distributional facts can be explained on the 

 supposition " that the parasite-species has been handed 

 down practically unchanged to the present . . . distinct 

 bird species from their common ancestor of earlier days." 

 In contrast with the specific differences between the 

 European and the American Coot or the European and the 

 American Avocet, '' the parasite of the common avocet or 

 coot ancestor of the two present bird species remains 

 unchanged, and is thus a single species com^mon to the two 

 geographically separated, never-meeting host species." In 

 many cases besides that of the Docophorus mentioned above, 

 birds of distinct genera and even of distinct families harbour 

 identical parasites. The most remarkable example of this 

 is afforded by the flightless birds (Ratitae or Struthiones) of 

 the great southern continents. On the South American 

 *' ostriches " (Rhea) live three species of Lipeurus, " one 

 being found on each of the two host species and the other 

 two on a third." One of these Rhea-haunting biting- lice 

 reappears on the true Ostriches of Africa, and another on an 

 Australian Cassowary. The significance of these associa- 

 tions is astonishing, for it follows certainly that these three 

 families of large flightless birds, so distantly related that 

 their common origin has been by competent ornithologists 

 denied and their likeness attributed to convergence, must 

 be derived from a common stock, on which the two species 

 of Lipeurus Hved in mid-Tertiary times and have since then 

 undergone no specific change. 



Since insects of many and varied types live as parasites 

 on large animals, it is not surprising to find that many 

 minute creatures live on or inside insects, and such life- 

 relations become especially interesting in cases where the 

 insect-host serves as a living link between the other two 

 organisms. The Mallophaga furnish a well-known example 

 of this connection in Trichodectes cam's, the biting- louse of 



