152 LIFE AND DEATH. [III. 



of host, change of medium, etc. It therefore results that the 

 fundamental law of biogenesis does not apply to the Monoplas- 

 tides ; for these forms are either entirely without a genuine 

 ontogeny and only possess the possibility of growth, or else 

 they are only endowed with a coenogenetic ontogeny^. 



Some authorities may be inclined to limit the above proposi- 

 tion, and to maintain that we must admit the possibility that 

 we are likely to occasionally meet with an ontogeny of which 

 the stages largely correspond with the most important stages 

 in the phyletic development of the species, and that the onto- 

 genetic repetition of the phylogeny, although not the rule, may 

 still occur as a rare exception in the Protozoa. 



A careful consideration of the subject indicates, however, that 

 the occurrence of such an exception is very improbable. Such 

 an ontogeny would, for instance, occur if one of the lowest 

 Monoplastides, such as a Moneron, were to develope into a 

 higher form, such as one of the Flagellata, with mouth, eye- 

 spot, and cortical layer, under such external conditions that it 

 would be advantageous for the existence of its species that it 

 should no longer reproduce itself by simple fission, but that 

 the periodical formation of a cyst (which was perhaps previously 

 part of the life-history) should be associated with the occurrence 

 of numerous divisions within the cyst itself, and with the forma- 

 tion of germs. We must suppose either that these germs 

 were so minute that the 3^oung animals could not 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 

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

 life. But the supposed transformation of the species also de- 

 pended upon these same conditions of life, which must there- 



^ Biitschli, long ago, doubted the application of the fundamental law 

 of biogenesis to the Protozoa (cf. ' Ueber die Entstehung der Schwarm- 

 sprosslings der Podophrya quadripartita,' Jen. Zeit. f. Med. u. Naturw. 

 Bd. X. p. 19, Note). Gruber has more recently expressed similar views, 

 and in fact denies the presence of development in the Protozoa, and 

 only recognizes growth (' Dimorpha mutans, Z. f W. Z.' Bd. XXXVII. 

 p. 445). This proposition must however be restricted, inasmuch as a 

 development certainly occurs, although one which is coenogenetic and 

 not palingenetic. 



