PARASITISM AND DEGENERATION 199 
active, keen-sighted, highly organized non-degenerate in 
free competition. But free competition is exactly what 
the degenerate animal has nothing to do with. Certainly 
the Sacculina lives successfully ; it is well adapted for its 
own peculiar kind of life. For the life of a scale insect, 
no better type of structure could be devised. A parasite 
enjoys certain obvious advantages in life, and even extreme 
degeneration is no drawback, but rather favors it in the 
advantageousness of its sheltered and easy life. As long 
as the host is successful in eluding its enemies and avoid- 
ing accident and injury, the parasite is safe. It needs to 
exercise no activity or vigilance of its own; its life is easy 
as long as its host lives. But the disadvantages of para- 
sitism and degeneration are apparent also. The fate of the 
parasite is usually bound up with the fate of the host. 
When the enemy of the host crab prevails, the Sacculina 
goes down without a chance to struggle in its own defense. 
But far more’ important than the disadvantage in such par- 
ticular or individual cases is the disadvantage of the fact 
that the parasite can not adapt itself in any considerable 
degree to new conditions. It has become so specialized, 
so greatly modified and changed to adapt itself to the one 
set of conditions under which it now lives; it has gone so 
far in its giving up of organs and body parts, that if pres- 
ent conditions should change and new ones come to exist, 
the parasite could not adapt itself tothem. The independ- 
ent, active animal with all its organs and all its functions 
intact, holds itself, one may say, ready and able to adapt 
itself to any new conditions of life which may gradually 
come into existence. The parasite has risked everything 
for the sake of a sure and easy life under the presently 
existing conditions. Change of conditions means its ex- 
tinction. 
106. Human degeneration.—It is not proposed in these 
pages to discuss the application of the laws of animal life 
to man. But each and every one extends upward, and can 
