No. 555] SPECIES-FORMING OF ECTO-PARASITES 135 



meet the group on the mate of this host, and these two 

 mingling groups may send their representatives or their 

 offspring to the young of the two mated hosts. But that 

 is about the extent of their participation in the life and 

 character of their species as a whole, and it is an extent 

 which plainly must result in the establishment of an 

 hereditary strain characterized by the special slight 

 structural idiosyncrasies peculiar to the few ancestors 

 from which the strain takes its origin. In the case of the 

 parasites of more gregarious bird kinds, as the seabirds 

 that mass for rest or brooding on ocean rocks or shore 

 cliffs, or the swallows and swifts that live in colonies in 

 caves and chimneys, or those gallinaceous birds like the 

 partridges of California that gather in close bands of 

 two or three score individuals, or others of any kind of 

 similar habit which may give chance for repeated actual 

 personal contact of body with body sufficient to permit of 

 migration of the wingless but active parasites from bird 

 to bird, this element of isolation is less accented. But 

 it still plays an important figure. For both the wingless- 

 ness and the manifest stay-at-home habits of the para- 

 sites make their movement from host to host at best a 

 desultory and almost accidental one. 



This combination of conditions, then, may serve to ex- 

 plain partly both why each species must be given a very 

 flexible description and why one might describe and name, 

 if he liked, many varieties of each species; and it ex- 

 plains, in some measure, why there are a good many 

 species in the order, and why there are many in each of 

 a few genera, although it does not explain, perhaps, why 

 there are some genera with very few, and indeed even, so 

 far as yet known, single species. 



The explanation of the actually small number of genera 

 and families depends, I think, upon one of the conditions 

 in the life of the Mallophaga which is directly opposed, 

 m its influence, in a way, to the isolation condition making 

 for a variation that results in numerous varieties and 

 s Pecies. It is this. Although the different host species 



