610 THE AMERICAN NATURALIST. [Vor. XXXIX. 
animals are without doubt the descendants of a long line of para- 
sitic ancestors and they probably represent the limit to which 
the habit can go. 
We are thus furnished with two criteria enabling us to gauge 
the modifications induced by this mode of life. That is, given 
two allied groups of parasites, that possessing the more degraded 
organization and more complex life history is to be considered 
as the less primitive. A third criterion is furnished by the part 
of the host chosen fora habitat. Without doubt, the original 
gateway was the mouth or the rectum. Thus the less modified 
parasites are inhabitants of the alimentary canal. Examples are 
such animals as Ameba blatte and the various Flagellates men- 
tioned above, which dwell in the recta of frogs and salamanders. 
Access to the lungs and liver is also easy, although some of the 
parasites of the latter organ reach their chosen place by an 
indirect path. In the case of the inhabitants of the sexual 
organs, muscles, and brain, the infection comes in most cases by 
way of the intestine. There is, however, no direct road and the 
parasite must bore its way through the tissues of the host. 
These parasites are generally of the most modified types. On 
the contrary, the greatly modified Cestodes, at least in one of 
their hosts, live in the alimentary canal. This criterion, then, 
needs to be used with much more caution than the other two. 
Having obtained certain criteria, I shall endeavor to apply 
them to the matter in hand, as indicated by the title of my com- 
munication. The Sporozoa stand alone. Their ancestors were 
two rival views regarding their origin. Minchin ! aptly terms 
these the euglenoid and the amoeboid hypotheses. The one 
regards the Flagellates, the latter the Rhizopods, as the ances- 
tral forms. Bütschli has advocated the former, whereas Min- 
Chin pronounces in favor of the latter. To me, however, it 
appears that neither can be adopted without reserve. Each, 
by implication, assumes that all Sporozoa have had a common 
14 Treatise on Zoölogy, edited by E. Ray Lankester, Chapter 1, Section K. 
