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 to them. 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 



