CONCLUDING SUMMABY: EXTENT OF BANGE OF VABIATION. 



575 



least account. It is in this type that we recognize the nearest approximation towards 

 such forms as Thalassicolla, which seem to connect Orbitolites with Sponges (^[ 67); 

 while the relationship which Orbiculina and Peneroplis have been supposed to bear to 

 the ordinary Helicostegues, being dependent only on plan of growth, and being utterly 

 at variance with the essential characters of the two groups, must be regarded as one of 

 analogy, not of affinity. Looking to the evidence already adduced in regard to the 

 prevalence of particular modifications of Orbitolites in particular localities (^[ 62), and 

 to the influence of the geographical distribution of the Peneroplis type upon the modi- 

 fications it presents (\ 138), we seem justified in extending the same view to those larger 

 (though not more essential) differentiations which these types must have undergone on 

 the hypothesis of their derivation from the same original. The following may be 

 suggested as the mode in which the existing forms might thus have diverged from 

 each other and from their primary type. 



Orbiculine Type 



diverging into 



A^ . 



Peneroplis 



Orbiculina 



. 

 Dendritiua, Peneroplis 



Orbiculina, Alveolina 



Dendritina, Spirolina, Pcneroplis. 



Orbiculina, Orbitolites, Alveolina. 



246. Passing on, now, to an essentially different group, that which includes Nummu- 

 lites and its allied forms, we find that the relation of the discoidal Cycloclypeus and the 

 helicine Hetcrostegina is of essentially the same nature with that of Orbitolites and 

 Orbiculina (^[^[113, 116); the minute structure of the shell and the physiological 

 condition of the sarcode-body being essentially the same in the two organisms, and the 

 only important divergence between them being in their plan of growth. From the 

 rarity of Cycloclypeus, all the known specimens of which have been brought from one 

 locality, I have not yet had the opportunity of ascertaining whether it ever presents in 

 an early stage any approximation to the helical mode of growth ; but such a deficiency 

 of affirmative evidence is obviously not equivalent to a disproof of what has strong analogy 

 in its favour. The variations which I have described among the different forms of Oper- 

 culina, although limited to the form of the spire and the character of the surface- 

 markings, would be amply sufficient to justify the erection of numerous species, were it 

 not for the connexion established between the most divergent forms by intermediate 

 links, and the necessity for an almost indefinite multiplication of hypothetical originals 

 which the adoption of such a method would involve. The existence of such a large 

 extent of variation among the specimens collected in the same locality must be admitted 

 as valid evidence of the facility with which differential characters develope themselves in 

 this type ; and a strong probability is thus afforded in favour of the varietal character 

 of larger differences among individuals whose conditions of existence are very diverse. 



