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ontogenic cycles. One cannot accurately speak of the "growth" 

 of a phylum, nor ought the word " development " to be used for 

 the phylum. Development should be restricted to the zoon or 

 individual or its morphic equivalent among Protozoa, since it 

 expresses more clearly the differences that exist between ontogeny 

 and phylogeny than their similarities, and for the same reason it is 

 advantageous to use evolution for the phylum alone in the sense in 

 which it is commonly employed. The necessity of subdividing the 

 embryonic stage is admitted, and in all probability this really 

 includes several stages with their own substages, but the discussion 

 of this problem must be left to the future. 



The paragerontic stage is in no sense "atavistic" or "rever- 

 sionary," as it is defined by Buckman and Bather. Reversions are 

 the returns or recurrence of ancestral characteristics in genetically 

 connected organisms which have been for a time latent in inter- 

 mediate forms. I do not think that we can include in this category 

 purely morphic characteristics which habitually recur in the same 

 individual as the result of paraplasis, or which occur in the paracme 

 of a type more or less invariably. In the individual the resem- 

 blance of the smooth round shell of the whorl of the paragerontic 

 ammonoid after it has lost the progressive characteristic of the 

 ephebic stage cannot be considered as a reversion. It is simply an- 

 alogy of form, not structural similarity of characteristics. A better 

 known and more easily understood case is the resemblance of the 

 lower jaw of the infant before it has acquired teeth and that 

 of the extremely old human subject in which these parts have been 

 lost and the alveoli and upper parts of the bony mandible have dis- 

 appeared through resorption. The forms are alike, but no one 

 would venture to consider the infant's cartilaginous jaw and that of 

 the old man as similar in structure. 



The best example of similar phenomena in the phylum known to 

 me is the close resemblance of form between the straight Baculites 

 of the Cretaceous or Jura and Orthoceras of the Paleozoic, which 

 has been described above, and is figured further on. One occurs 

 in the paracme and the other in the early epacme of the group of 

 chambered shells. They are widely distinct in their structural 

 characteristics, and these differences are greater in the young than 

 at any subsequent stage of their ontogeny, Baculites having a close- 

 coiled shell in the nepionic stage, and Orthoceras is straight from 

 the earliest stage. The return of a similar form in Baculites in the 



