42 OF THE FOEAMINIFERA GENERALLY : 



as to be easily separable from each other by accidental violence, and of which the animals 

 can maintain their lives jnst as well when they are thus broken up into distinct segments 

 as when retaining their original connexion, such may be regarded as poientially Mono- 

 thalamous ; and the fact that the segments of sarcode, as they were successively budded forth 

 from the stock, formed their shelly investments before instead of after their detachment from 

 it, can scarcely be admitted by the physiologist as alone justifying an ordinal differentiation 

 which is not borne out by other structural or physiological diversities. 



53. It was not unnatural that, in seeking for a basis on which to found an arrangement 

 of the multitudinous forms of Foraminifera which he for the first time brought together under 

 one distinct category, M. D'Orbigny should have attached primary importance to characters 

 so easily recognised as those which are produced by diversities in the plan on which the suc- 

 cessive segments are added one to another. For the varieties of form thus produced seem at 

 first sight easily capable of being reduced to a small number of primary types ; and it is only 

 by such a laborious and conscientious comparison of osculant forms as formed no part of M. 

 D'Orbigny 's method of study, that the essential conformity in plan of growth is discovered 

 which often exists between organisms arranged by him under different orders ; whilst it is 

 only by an equally painstaking examination of the internal structure of the shell, such as seems 

 never to have been even thought of by M. D'Orbigny, that those very marked characters are 

 brought to light, which often separate by the widest interval organisms grouped by him under 

 the same order, and which bring these respectively into intimate relationship with others 

 whose place in his series is very remote. Thus, when we come to speak of the genus 

 CristcJlaria, we shall find that it comprehends a series of straight, curved, and spiral forms, 

 agreeing with each other in all essential particulars save the direction of their axis of growth, 

 and presenting such a continuity in the gradation from the straight to the curved, and from 

 the curved to the spiral, as prevents any decided line of demarcation from being anywhere 

 drawn among them. So, again, we shall find that although, on the ground of conformity in 

 their plan of growth, Orbitolites and Cydochjpeus would be grouped together by M. D'Orbigny 

 in his order Ci/closter/ues, whilst Oriiculiiia, Penerojilis, and Operculina are placed in his order 

 Helicostef/ues, and Heferostegina in his order Enfomostef/ues, a careful comparison of the 

 essential features of their structure shows that not only have Orbitolites and Ci/clocli/peas 

 nothing in common but their cyclical mode of growth, but that Orbitolites is most intimately 

 related to Orbiculina (which often takes-on the cyclical mode of growth), and through it to 

 Peneroplis, whilst CyclocJypeus is scarcely less intimately related to Heterosteyina and through 

 it to Operculina. It may, in fact, be most safely asserted, thatj'Vaw of groicth is no more to be 

 regarded as an exponent of the really natural afiinities of the several generic types of 

 Foraminifera, than the number of stamens and pistils is of the natural affinities of Phanero- 

 gamous plants. The system founded upon each of these bases will doubtless, in many 

 instances, bring together types which have a real affinity to each other, simply because the 

 characters in question sometimes coincide with those of more essential value ; but such coin- 

 cidence is (so to speak) accidental ; and it much more frequently happens in the one as in 

 the other of these artificial systems, that they separate by a wide interval types which in 

 reality are closely related, whilst those which they bring into nearest proximity are essentially 

 diverse in organization. 



