SOCIAL AND COMMUNAL LIFE 421 



special sense organs. The males lead a free active life, 

 but the females have nearly or quite given up the power 

 of locomotion, attaching themselves by means of their 

 sucking beak to some plant, where they obtain a suffi- 

 cient food-supply (plant-sap) and lay their eggs. In both 

 males and females the larvae are little active crawling six- 

 legged creatures with legs, eyes, and antennae. 



We are accustomed perhaps to think of degeneration 

 as necessarily implying a disadvantage in life. It is true 

 that a blind, footless, degenerate animal could not cope 

 with the 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 and the scale-insects live 

 well ; they are admirably adapted to the kind of life they 

 lead. A parasite enjoys certain obvious advantages in life, 

 and even extreme degeneration is no drawback (except as 

 we shall see later), but gives it a body which demands less 

 food and care. As long as the host is successful in elud- 

 ing its enemies and avoiding accident and injury the para- 

 site is safe. Its life is easy as long as the host lives. 

 But the disadvantages of parasitism and degeneration are 

 nevertheless obvious. The fate of the parasite is 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 defence. But far more im- 

 portant than the disadvantage in such particular or indi- 

 vidual cases is the fact that the parasite cannot adapt itself 

 in any considerable degree to new conditions. It has 

 become so modified, so specialized to adapt itself to the 

 very special conditions under which it now lives, it has 

 gone so far in giving up organs and functions, that if 

 present conditions change and new ones come to exist 

 the parasite cannot adapt itself to them. The independ- 

 ent free-living animal holds itself, one may say, able and 



