ORIGINS OF parasitism: evolution of parasites 53 



fish have dwarf males which hve on them as parasites and many 

 molluscs are hermaphrodites. Snakes and earthworms have lost their 

 legs or ambulatory processes and certain jelly-fish are capable of 

 absorbing nourishment through their skins. In these cases the factors 

 which determine a vast output of eggs, the development of suckers, the 

 loss of organs of locomotion and so forth, obviously have nothing to do 

 with a parasitic environment. It is easy to see that any animal might 

 already possess one or several of these specialisations before it took to the 

 parasitic mode of life. Furthermore, the difficulty experienced, say, in 

 reaching a new host is obviously so great that it is hard to conceive how 

 such an animal could become established as a parasite unless it already 

 possessed immense powers of reproduction. The theory of pre-adapta- 

 tion is, therefore, undoubtedly attractive in the case of species which are 

 introduced suddenly and violently into an entirely new environment. 



It is as well to remember that if individuals vary at all pre-adapta- 

 tion must exist. Some of our friends seem to enjoy heat waves, while 

 others become inactive and sit around sighing and mopping their 

 brows. It is permissible to claim that the former are pre-adapted to 

 a hotter climate. Professor J. B. S. Haldane once demonstrated at a 

 Royal Society Conversazione that he is immune to the bite of bed bugs. 

 So was his father. The Haldane family are therefore pre-adapted to 

 survive a bug-borne epidemic, should one occur. To say that plants and 

 animals may be pre-adapted is really only another way of saying they 

 are not all alike, for every variation is potentially both adaptive and 

 pre-adaptive. On the other hand, as Bodenheimer has pointed out, in 

 one sense no real adaptation to a new environment ever takes place. 

 No matter how different life may be, say, in the sea, or in the gall- 

 bladder of a bird, an animal's response cannot surpass the hereditary 

 base of reactions. Theoretically evolutionary possibilities are endless, 

 but certain lines are mutually exclusive, and once an animal has 

 started along one of these evolutionary paths, others are automatically 

 barred. The more highly specialised an animal becomes the less are 

 its chances of being able to break away, and certain lines are thus self- 

 directing and self-restricting. Therefore, although Sacculina develops 

 root-like extensions of the body which ramify throughout the host's 

 tissues, it cannot turn into a plant — although such a transformation 

 might have definite advantages. 



There are also some very interesting examples of direct modifica- 

 tions which have been produced merely by a change of host. For 



