274 Cal'ses and Course of Organic Evolution 



tribution of chromatin during these processes also indicates 

 that repulsive lines of energy-flow traverse the chromatin 

 substance. As amongst these algse, free cell formation, with 

 subsequent production of amoeboid or ciliate swarmspores, 

 is a frequent occurrence. Here also it has been demonstrated, 

 in at least some instances, that such formation is due to envir- 

 onal changes either in food supply, in composition of the sur- 

 rounding water, in exposure to dessication conditions, or in 

 relation to some host. Further, as among the simpler algse, 

 such free cell-formation may^ only result in the setting free 

 of numerous spore-cells, each capable of developing into a 

 new individual. Amongst some Radiolaria even, as amongst 

 algse already cited, one crop of swarmspores — the so-called 

 isospores — may directly give rise to new individuals, while 

 another — the anisospores — require to conjugate for formation 

 of new individuals. 



As to the origin and occurrence of sexuality amongst the 

 Protozoa, this question seems to be largely bound up with 

 the possible origin of the entire group from some more primitive 

 ancestral types. Tliis involves again the question of the 

 possible monophyletic or polyphyletic descent of the group. 

 If, as advocated in a succeeding chapter (XII), we view some 

 of the autotrophic or the saprophytic bacteria as ancestral 

 forms, these are still regarded as having no definite nucleus 

 and no sexual differentiation. It is by no means unlikely 

 therefore that organisms which have been classed in the divi- 

 sion Proteomyxa are alike anucleate and asexual, or show 

 only evolving nuclear and sex differentiation. Thus in Gloid- 

 ium, Protamoeba, etc., careful study has hitherto failed to 

 reveal nuclei, in Tetramyxa only scattered chromidia occur, 

 in Eudyonema the organism is devoid of "nuclear apparatus" 

 during the vegetative phase, but the aggregation of chromatin 

 into a nucleus takes place when reproduction is about to occur, 

 while in Psevdospora and Nuclearia there is a definite nucleus. 



But, given the formation of a nucleus, sex-differentiation 

 seems then gradually but invariably to appear. As already 

 pointed out also (p. 264) there are definite proofs amongst 



