INTRODUCTION 17 



parasite as a means at once of fructification, and of dispersal 

 that is both ample and exact, that the biological idea of a 

 carrier should be reserved. 



Still it is evident that no hard-and-fast line of separation 

 can be drawn between such a fixed and constant engine of 

 distribution, and a casual and uncontrived general-carrier 

 such as a fly. 



If at the first blush there seems to be something entirely 

 strange and new, not so much in the fact that a blood-sucking 

 insect can imbibe a parasitic animalcule from one individual 

 victim in one shape and can inject it into another individual 

 victim in another shape, as in the fact that the very existence 

 of the said parasitic animalcule as a species depends upon 

 this roundabout and precarious process, it has to be remem- 

 bered that the component features of the phenomenon are 

 by no means isolated or unique. 



It is a familiar experience, for instance, that lowly 

 organisms of many kinds may often score a point in the 

 internecine struggle for existence, by that extreme form of 

 passive resistance known in popular language as going to 

 bed, or in technical language as encystment. Safe in its 

 shell it lets the world slip, as the Baron of Sheppey in the 

 Ingoldsby Legend treated the Sheriff of Kent and his posse. 



Again, it is a familiar experience that an encysted 

 organism may merely remain latent until its opportunity 

 arrives ; like the spore of a bacillus, like an encysted amoeba, 

 or like the gemmule of a Sponge or the statoblast of a 

 Polyzoon. Or it may mechanically adhere to some other 

 active animal and itself remaining inert, may be transported 

 to some happy hunting-ground where it resumes the ordinary 

 tenour of its life ; as the quiescent " Hypopus " of certain 

 mites is transported by flying insects, or as the gemmule 

 of a sponge may be transported by birds. Or a paired 

 and encysted couple may, within the wall of its cyst, break 

 up into a multitude of independent particles, like an 

 encysted Gregarinid or Coccidiid ; each of the latent units 

 being ever ready, when liberated from confinement, 



" To take occasion by the hand 

 And grasp the skirts of happy chance." 



B 



