III.] LIFE AND DEATH, 1 53 



fore have been of such a nature as to bring about simultaneous!}'-, 

 by an intercalation of germs and by a genuine development, 

 the evolution of the form in question in the last stage of its 

 ontogeny, and the maintenance of its original condition during 

 the initial stage. Such a combination of circumstances can 

 have scarcely ever happened. Against the occurrence of such 

 a transformation as we have supposed, it might be argued, in- 

 deed, that the assumed production of very numerous germs 

 does not occur among free-living Monoplastides. Those which 

 have acquired parasitic habits must be younger phyletic forms, 

 for their first host— whether a lowly or a highly organized 

 Metazoon — must have appeared before they could gain access 

 to it and adapt themselves to the conditions of a parasitic life, 

 and by this time the Flagellate Infusoria were already estab- 

 lished. It is by far less probable that the persistence or rather 

 the intercalation of the ancestral form would occur in an onto- 

 genetic cycle, consisting of a series of stages, and not of two 

 only, as in our example. For as soon as reproduction can be 

 effected by the simple fission of the adult, not only is there no 

 reason why the earlier phyletic stages should be again and 

 again repeated, but such recapitulation is simply impossible. 

 We cannot, therefore, conclude that the anomalous early stages 

 of a Monoplastid such as A cine fa correspond with an early form 

 of ph3detic development. 



Supposing, for instance, that the Acinetaria were derived 

 from the Ciliata, then this transformation must have taken 

 place in the course of the continued division of the ciliate an- 

 cestor — partially connected with encystment, but for the most 

 part independently of it. Of the myriads of generations which 

 such a process of development may have occupied, perhaps the 

 first set moved with suctorial processes, while the second 

 gradually adopted sedentary habits, and throughout the whole 

 of the long series, each succeeding generation must have been 

 almost exactly like its predecessor, and must always have con- 

 sisted of individuals which possessed the characters of the 

 species. 



This does not exclude the possibility that in spite of an as- 

 sumed sedentary mode of life, the need for locomotion and for 

 obtaining food in fresh places may have arisen at some period 

 of hfe. But whenever formation of swarm-spores takes place 



