186 COMPARATIVE ANATOMY 



mitted by the bites of ticks. P. cam's, also transmitted by 

 ticks, produces malignant jaundice in dogs, and P. equi is the 

 cause of a fever similar to malaria in horses. These malignant 

 and unfortunately too common little organisms have only 

 been discovered and studied in recent years, and the life 

 histories of many of them are still imperfectly known. They 

 are so minute in size that the highest powers of the microscope 

 and the most refined methods of histological technique are 

 necessary for the elucidation of their structure. But it must 

 not be supposed that because of their littleness they are not 

 included in the domain of Natural History. In truth they 

 form one of the most interesting chapters in the study of 

 nature, and afford a striking example of those intimate relations 

 between different animals which have been acquired, step by 

 step, in the course of organic evolution, and by no means 

 always to the benefit of all the animals concerned. It would 

 be superfluous to insist on the practical importance of studies 

 which enable us to cope successfully with some of the most 

 virulent and dreaded diseases of man and domesticated 

 animals. 



" Aib Se? fj,r) Svcr^epaivcLV TrcuSiKws Trjv Trepi TMV a 

 TTia-K\f/iv. ei' Tracrt yap TOI? (J>VCTLKOLS 



* " Wherefore one should not be childishly contemptuous of the study of 

 the most insignificant animals. For there is something marvellous in all 

 natural objects." ARISTOTLE, De Partibus AminaHum, i. 5. 



