394 THE PROTOZOA 



cell- parasites of the second (compare Leger and Duboscq, 646). However 

 enticing such a view may seem when only the forms parasitic in the verte- 

 brate hosts are taken into consideration, the facts of the development in the 

 invertebrate hosts must dispel completely any notion of affinity between 

 the two types. Nothing could be imagined more different than the develop- 

 ment of Leishmania, with its typical leptomonad forms (Fig. 140), and that 

 of Piroplasma (Fig. 162), with no flagellated stages at all in its life-cycle. It 

 becomes evident at once that any apparent resemblance between the two 

 genera is due to convergent adaptation induced by a similar mode of parasitism, 

 and that the two forms are in reality poles apart, with no more real affinity 

 than porpoises and fishes, or bats and birds. It is certainly not at this 

 point that any transition from one group to the other is to be sought.* 



In the foregoing paragraphs an attempt has been made to sum 

 up the arguments for and against the theory that the Hsemosporidia 

 are to be removed from the vicinity of the Coccidia, and classified 

 with the trypanosomes and allied forms in an order of the Flagellata. 

 When the evidence on each side is weighed in the balance, in one 

 scale must be placed the complete similarity of the life-cycles of 

 typical Coccidia and Hsemosporidia, a similarity seen in every phase 

 of the life-cycle, and extending even to minor developmental 

 details ; and in the other scale certain cell-granules of doubtful 

 significance. It is almost inconceivable that more importance 

 should be attached to cytological details, the genetic and classifi- 

 catory value of which is at present quite uncertain, than to the 

 homologies of the life-cycle as a whole, in estimating the affinities 

 of the orders of Protozoa ; the more so since even in the HoBmo- 

 flagellates themselves the possession of the binucleatc type of 

 structure does not, apparently, indicate a common ancestry for all 

 members of the group. 



The conclusion reached is, then, that the Hsemosporidia as a 

 group, excluding doubtful forms insufficiently investigated at 

 present, are closely allied to the Coccidia. It is, indeed, probable 

 that there are two lines of evolution in the group the one repre- 

 sented by the haemamcebse, halteridia, and true leucocytozoa, 

 descended from a Coccidium-like ancestor ; the other represented 

 by the haemogregarines, from an ancestral form similar to Adelea 

 or Orchedbius. Leger (644) has classified the hsemogregarines in 

 the section Adeleidea of the Coccidia, and one may regret that 

 the distinguished French naturalist did not go one step farther and 

 place the haemamcebaa in his section Eimeridea (see p. 352, supra). 



On the other hand, any resemblances which the Haemosporidia 

 exhibit to trypanosomes and allied forms are due to convergent 

 adaptation on the part of the Flagellates themselves, and more 

 especially to the secondary acquisition by the latter of intracellular 



* Leger and Duboscq (646), who derive Leishmania and Babesia directly from 

 Crithidia as a common ancestor, do not seem to have taken the development of 

 Babesia (Piroplasma) into consideration at all ; they neither refer to it in their 

 text nor cite any of the relevant memoirs in their bibliography. 



