LIFE AND DEATH. 151 



become Flagellata directly, or that it was advantageous for them to 

 move and feed as Monera at an early period, and to assume the 

 more complex structure of the parent by gradual stages. In other 

 words, the phyletic development would proceed hand in hand with 

 the ontogeny corresponding to it, although not from any in- 

 ternal cause, but as an adaptation to the existing conditions of 

 life. But the supposed transformation of the species also depended 

 upon these same conditions of life, which must therefore have been 

 of such a nature as to bring about simultaneously, by an inter- 

 calation 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, indeed, that the assumed pro- 

 duction 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 established. It is by far less probable that the persistence 

 or rather the intercalation of the ancestral form would occur in an 

 ontogenetic cycle, consisting of a series of stages, and not of 

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

 eifected 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 Acineta correspond with an early form of 

 phyletic 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 ancestor partially 

 connected with encystment, but for the most part independently of 

 it. Of the myriads of generations which such a process of develop- 

 ment 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 



