450 The American Naturalist. [May, 
and results of parasitism. 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 non-parasitic 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 ulti- 
mately 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, 
Acaride, 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 specialization of parts with reference 
to this habit will be observed. The disuse of certain organs, as wings 
for flight and feet for ordinary locomotion, results in reduction or modi- 
fication of these organs, and hence we find almost invariably that para- 
sitic species are wingless, and that they have the feet adapted specially 
for locomotion among the hairs or feathers of the hosts. This adapta- 
tion 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 cannot 
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 assumes a parallel char- 
acter in widely divergent groups, as the clasping organs developed 1m 
