150 The Naturalist in La Plata. 



for blood and the power to draw it, and ready at 

 any moment to return to the ancestral habit. It 

 might be said that if such a result were possible 

 it would have occurred, but that we find no insect 

 like the Ornithomyia existing independently. With 

 the bird-fly it has not occurred, as far as we know ; 

 but in the past history of some independent para- 

 sites it is possible that something similar to the 

 imaginary case I have sketched may have taken 

 place. The bush-tick is a more highly specialized, 

 certainly a more degraded, creature than the bird- 

 fly, and the very fact of its existence . seems to 

 show that it is possible for even the lowest of the 

 fallen race of parasites to start afresh in life under 

 new conditions, and to reascend in the scale of 

 being, although still bearing about it the marks of 

 former degeneracy. 



The connection between the flea and the mammal 

 it feeds on is even less close than that which exists 

 between the Ornithomyia and bird. The fact that 

 fleas are so common and universal for in all lands 

 we have them, like the poor, always with us ; and 

 that they are found on all mammals, from the king 

 of beasts to the small modest mouse seems to show 

 a great amount of variability and adaptiveness, as 

 well as a very high antiquity. It has often been 

 reported that fleas have been found hopping on the 

 ground in desert places, where they could not have 

 been dropped by man or beast; and it has been 

 assumed that these "independent " fleas must, like 

 gnats and ticks, subsist on vegetable juices. There 

 is no doubt that they are able to exist and propagate 



