PROBLEM I »..., 391 



bi^t because they are so commonly variable.. Any set of specimens taken 

 from the same beds varies greatly in, form, an,d this plasticity of form is 

 not confined to samples taken in any special stage of its long history* or 

 any .particular locality of its wide distribution. 



On the other hand, a species like the Stroplionella funiculata presents 

 slight degree of variation in its specific characters and is itself a- special 

 phase, of the more widely variable Stroplionella euglypha. In wljat are 

 regarded its generic as well as its specific characters, specimens found 

 associated in a single fauna or a single locality are not markedly different 

 and hence fall under a more rigid definition. In the case of along-liyed 

 species like Strophomena rhomb oidalis, which retains its morphologic^ 

 plasticity, the conclusion seems reasonable that it lived and flourished 

 continuously somewhere throughout the whole time represented by the 

 four geological systems in which it is found, and that its, presence or ab- 

 sence in any particular formation of those systems was determined by 

 favorable or unfavorable conditions of environment, and therefore ca,n 

 not be taken as evidence of contemporaneity of the particular formations 

 holding the species except within the limits of its known existence in the 

 series. 



, In ;the other extreme case of a short-lived species like Stroplionella 

 funiculata, which in any particular section is known to have a short ver- 

 tical range and to preserve its specific characters rigidly, we are led to a 

 similar conclusion. If we start with the assumption that its peculiar 

 morphologic characters are the reflex response of the organism to par- 

 ticular conditions of environment, wliat is there to prevent the recurs 

 rence of such response in the race to which it belongs at any recurrence 

 of the appropriate conditions? If; on the otber hand, we assume that it 

 was a phase of race-evolution, not being capable of adapting itself in :i .the 

 struggle for existence and therefore closely adjusted to a particular set 

 pf environmental conditions, the meeting of the same species in rocks of 

 two widely separate regions, such as Maine and the center of England, 

 calls for time to migrate, and the further apart the locality of the faunas 

 the longer the time interval separating them. In neither of these ex- 

 treme cases are we safe in inferring identity of time from the presence 

 of identical species, and most of the species of a fauna are intermediate 

 between these extreme examples. 



C. Personal equation of the describer of species.— J&wt these are not the 

 only problems involved in interpreting identity of the species- of two 

 faunas into contemporaneity of the formations containing them?. 



