16 INSECTS AFFECTING DOMESTIC ANIMALS. 
general outline, but it is a general rule that they have but three pairs 
of legs, whereas the adults as a rule have four. When gorged with — 
blood the adults may become distorted in shape, when the resemblance 
to the young becomes less marked. 
ORIGIN OF THE PARASITIC HABIT. 
The problems of the origin of parasites, or the adaptation of certain 
forms to a parasitic life, are among the most interesting met with in 
biological investigation, but we can suggest merely some of them here. 
It may be said from the biological standpoint that all parasites have 
been derived primarily from nonparasitic forms—a proposition which 
is supported by innumerable facts in their morphology and embryology, 
and which may also be argued deductively. Since many species are 
confined absolutely to certain animals as hosts, it is evident that they 
could not have existed as parasites upon such species at least before 
the occurrence of the host. Unless, therefore, we claim an independent 
origin‘for them subsequent to the origin of the host we must allow an 
adaptation from some free-living species or from a parasitic species 
on some other host, and following this back for its origin, we must 
ultimately arrive at a free form as the source. 
In many cases the line of evolution is very apparent, as, for instance, 
the gradation between comparatively free and fixed Mallophaga, 
Acaridz, Pulicide, ete. 
RESULTS OF PARASITISM, 
It is also interesting to inquire as to the effect of the parasitic life 
upon the parasite itself. 
The natural tendency of an animal once started in the direction of 
parasitism will be to become more and more parasitic in habit, and 
with this habit a greater and greater snecialization of parts with 
reference to this habit will be observed ‘he disuse of certain organs, 
as wings for flight and feet for ordinary Iccomotion, results in reduction 
or modification of these organs, and hence we find almost invariably 
that parasitic species are wingless, and that they have the feet adapted 
specially for locomotion among the has or feathers of the hosts. This 
adaptation is often looked upon as degradation; but it seems to me 
preferable to consider it as a limitation in certain directions with 
specialization of certain organs. We consider the foot of the horse 
highly specialized, and we must admit that the animal is limited in its 
use, aS it can not climb trees, but we do not call the horse degraded. 
It is true that the limitations for many parasites are so great that 
they are absolutely dependent upon certain hosts, and the presence of 
certain conditions for their existence—there is reduction or degradation 
of certain organs, but progressive specialization of those organs which 
remain functional. Often such specialization assi.mes a parallel char- 
acter in widely divergent groups, as the clasping urgans developed in 
