ORGANIC ADAPTATION 339 



represented by the relation that occurs between Ants and Plant 

 Lice, or Aphids. The defenseless Aphids are protected, herded, and 

 'milked' by the Ants to supply their demand for honeydew, a 

 secretion of the Aphids which the Ants greedily devour. (Fig. 222.) 



3. Parasitism 



But associations in which one organism, the parasite, secures 

 the sole advantage, and in most cases at the expense of the help- 

 less second party, the host, are far more numerous — it has been 

 estimated that nearly half the animal kingdom are parasites. And 

 these justly receive considerable notoriety because many human 

 diseases are the result of Man's unwilling partnership in such 

 associations. Indeed, parasitology has become an important 

 subdivision of biology, both practical and theoretical. Practical, 

 as a cornerstone of public health; and theoretical, because many 

 of the most remarkable functional and structural adaptations are 

 exhibited by parasites in becoming fitted for this apparently 

 highly successful method of gaining a livelihood, and by the hosts 

 in bearing the burden with the least outlay. Generally speaking, 

 the effect on the parasite consists in a simplification of the various 

 organs of the body devoted to food-getting, locomotion, etc., 

 since their duties are performed by the host; while the organs and 

 methods of reproduction are highly specialized and elaborated, 

 owing to the necessity of producing enough offspring to compen- 

 sate for the hazards involved in reaching a proper host. For in 

 the majority of cases a parasite is adapted to live in a specific 

 host, and death ensues if this is not attained at the proper time. 

 (Figs. 25, 251, 252, 266.) 



Probably the most generally interesting example of parasitism 

 is the cause of the disease known as malaria. Man is subject to 

 at least three types of malaria, each the result of infection by a 

 different malarial organism. The malarial parasites are all uni- 

 cellular animals, Protozoa, with complicated life histories which 

 are adaptations to the specific demands of their parasitic exis- 

 tence. One part of the life history, the asexual, is passed in the 

 red blood corpuscles of Man; while the other, the sexual, occurs 

 in the digestive tract of certain species of Mosquitoes. A single 

 parasite inoculated into the human vascular system by the bite 

 of an infected Mosquito enters a red blood corpuscle and multi- 

 plies. The progeny, liberated from the destroyed corpuscle, sim- 



