THE H^MOFLAGELLATES. 155 



appear to be widely distributed among tlie principal classes 

 o£ Vertebrates; and^ at tlie present time, hardly a month 

 passes without a new host being added to the list. It is more 

 difficult to be certain with regard to the frequency with which 

 individual species of the parasites occur, the data being, as 

 yet, somewhat scanty. In one or two instances, however, they 

 are known to be of fairly common occurrence, the Trypano- 

 soma lewisi of rats, for example, being quite as abundant 

 as many Gregariues. This form is met with in all parts of 

 the world, having accompanied the Rodents in their ubiquitous 

 migrations. The proportion of hosts infected varies usually 

 from 10 per cent, to 30 per cent., according to the locality, 

 but, in Berlin, Rabinowitsch and Kempner have found that it 

 may be as high as 41 per cent. 



Parasitism in General. — Another reason accounting for 

 the comparative neglect of the Trypauosomes has been the 

 fact that the forms prevailing throughout the greater part o£ 

 Europe are non-pathogenic — that is, they do not, under 

 ordinary circumstances, give rise to any obviously harmful 

 effects in the animals which harbour them. Attention has 

 not therefore been directed to them by anything comparable, 

 for example, to the devastating epidemics of coccidiosis or 

 myxosporidiosis which sometimes occur. 



Animals liable by their natural distribution to the attacks 

 of a given parasite may be divided into two classes according 

 to their behaviour towards it. Either they are imnnme — this 

 term being used to signify that the attacked animal is actively 

 repellent^ to the parasitic organism, which is thus unable to 

 gain a footing — or they are susceptible. The reaction be- 

 tween any given parasite and its host, in the latter case, may 

 be regarded as the resultant of several factors. The host, on 

 its part, in many, perhaps in most instances has become 

 accustomed or inured to the invader, and is, apparently, 

 practically indiffei'ent to its presence. Again, to consider 



^ The terms "repellent" and "tolerant" are suggested by Laukester 

 ('Quarterly Review,' July, 1901) in his interesting discussion of the bio- 

 logical relations between a parasite and its host. 



