368 EVOLUTION AND ANIMAL LIFE 



competition is exacth^ 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 avoiding 

 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 parasitism and degenera- 

 tion 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 particular or individual cases is the 

 disadvantage of the fact that the parasite cannot 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 

 present conditions should change and new ones come to exist, 

 the parasite could probably not adapt itself to them. The 

 independent, active animal with all its organs and all its func- 

 tions 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 con- 

 ditions. Change of conditions means its extinction. 



