POLYMORPHISM AND LIFE-CYCLES 185 



vertebrate host to another, it is probable that it could dispense alto- 

 gether with the sexual cycle, which occurs only in the invertebrate 

 host, so far as is known. In the suborder Eugregarinse of the 

 Gregarinoidea an opposite condition occurs, since these forms 

 possess only the sexual cycle, sporogony, and there is no non-sexual 

 schizogony. Whether this condition is to be regarded as a primitive 

 state of things, or whether the Eugregarines are to be regarded as 

 having dispensed with the non-sexual process of schizogony seen 

 in the allied suborder Schizogregarinse, must remain an open 

 question. 



A further caution is also necessary with regard to the alternation 

 of generations in Protozoa. From the known facts of the malarial 

 life-cycle, in which an alternation of sexual and non-sexual cycles 

 is correlated with an alternation of hosts, it has often been assumed, 

 implicitly or explicitly, that a similar alternation of sexual and non- 

 sexual cycles must occur in other cases where there is an alternation 

 of hosts, as in the case of trypanosomes, and in particular that the 

 sexual cycle must occur in the invertebrate host. This assumption 

 is by no means justified, however, and has been the cause of much 

 unsound or unwarranted interpretation of the facts, especially as 

 regards the significance of the various forms of trypanosomes, 

 which are continually ascribed to sexual differentiation on no other 

 ground than the bare fact of form-differentiation, as pointed out in 

 the previous chapters. Up to the present there is not a single case 

 in which sexual phenomena in trypanosomes have been described 

 in a perfectly satisfactory manner, free from all doubt ; and, on the 

 other hand, it has been asserted that the syngamy occurs in the 

 vertebrate host in these parasites (Ottolenghi, 492). 



Bibliography. $ 'or references see p. 480. 



