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 lfe 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 
enats and ticks, subsist on vegetable juices. ‘There 
is no doubt that they are able to exist and propagate 
