J. IMBRIE 131 



morphological change within the sampled time interval cannot 

 be demonstrated. Tims species recognized on time planes A, B, 

 and C by the paleontologist represent stages in an evolutionary 

 continuum and are thus analogous to neontological species on 

 plane D. In some rather rare instances the paleontologist can 

 assemble material representing time planes so closely spaced that 

 modal shifts in an evolving succession of populations can be 

 demonstrated. Such circumstances are relatively rare, however, 

 and the paleontologist usually deals with widely spaced cross sec- 

 tions of evolving lineages. The term transient species will be used 

 in this paper to identify such cross sections, i.e., reproductively 

 isolated groups of individuals living during a single instant of 

 geological time. Normally the paleontologist's cross section is 

 limited geographically, and when this is the case, the transient 

 species of the paleontologist corresponds to the nondimensional 

 species of the neontologist (Mayr et at, 1953). In an increasing 

 number of instances, however, paleontologists have been able to 

 document taxonomically important infraspecific geographic dif- 

 ferences, i.e., polytypic transient species. 



For practical reasons the neontologist can rarely undertake the 

 field sampling and breeding experiments required to document 

 statistically significant seasonal shifts in gene frequencies. Hence 

 the paleontologist of necessity and the neontologist by default 

 describe transient species in the majority of cases now on record. 



In favorable circumstances the paleontologist can obtain ma- 

 terials which enable him to document gradual morphological 

 changes in a succession of fossil populations. In Fig. 2, for ex- 

 ample, collections taken at a number of horizons might define the 

 course of evolution between transient species 4 and 9. Quite 

 naturally, paleontologists use the term species for such segments 

 of phyletic lineages. Lineage segments of this sort can be called 

 successional species to distinguish them from the static groupings 

 referred to above as transient species. A number of theoretical 

 taxonomic and nomenclatural problems which arise in defining 

 the scope of successional species will be considered below in 

 connection with a discussion of Micraster. 



