608 THE AMERICAN NATURALIST. [Vor. XXXIX. 
Finally, a parasite acquires the ability to resist being digested. 
This appears in some cases to result from the formation of a 
differentiated external layer, but in general it is to be credited 
to some property of the living protoplasm. It is, necessarily, a 
property possessed by all organisms which live within others, 
and hence has no taxonomic value. 
But if the existence of the individual is rendered easier, that 
of the species is menaced by a danger not experienced by free- 
living forms. With these, birth, growth, and reproduction all 
take place in free nature. For the most part, the eggs and 
young are left to shift for themselves, but the dangers which 
they encounter, are much the same as those which threaten the 
adults. The greater mortality during these stages is primarily 
due to their greater helplessness. But it is far different in the 
case of parasites. For these the course must, so to speak, -con- 
sist of a series of leaps. A given host, at best, eventually dies 
and its death involves that of its parasites, as individuals. 
Hence the eggs or spores must be transferred to another host 
or else the species will cease to exist. Parasites are so abun- 
dant, and so thoroughly well established that we are prone to 
overlook what a radical departure this is from the ordinary 
scheme of animal life. The elements destined to tide the spe- 
cies over this critical period reach the exterior in one of several 
ways. The more usual are either by the death of the host, or 
by being carried out with its fzeces. Ordinarily animals do not 
eat either their own dead or their own fæces, whence the only 
chance the germ has is to become attached to the food and so 
swallowed. Accordingly, as a general rule, parasites are com- 
mon in social and uncommon in solitary animals. The termites, 
which do eat their fzeces, invariably have their intestines literally 
packed with the curious Trichonymphide. 
The response of the species to this acquired peril is two-fold. 
Complications are introduced into the life history. These are of 
such a character as to increase the probability of obtaining lodg- 
ment within the appropriate host. This is the most striking 
feature displayed by parasites. The other response is the acqui- 
sition of great fecundity. Great stress is always laid upon this, 
yet it is after all doubtful if parasites, taken as a whole, do. 
