THE FORAMINIFERA 137 



In Rhumbler's view " Festigkeitsauslese," the selection of the forms 

 of test best adapted to resist mechanical stress, is regarded as the chief 

 factor which has dominated the differentiation of the Foraminifera, 

 and several series of genera, such as the Nodosaria-Cristellaria and the 

 Biloculina-Quinqueloculina series, are given as examples of a " Festigkeit- 

 skala " in which varying degrees of resisting power have been attained. 



In the biformed and triformed tests the early chambers are regarded 

 as arranged on a higher (i.e. more resisting) plan than those added later, 

 and hence it is concluded that in the ontogeny of the Foraminifera the 

 order of the appearance of the more primitive and the later acquired 

 characters is the reverse of that so general in the development of other 

 animals, the earlier arrangement representing the form towards which 

 the race is advancing, the later retaining the characters which will ulti- 

 mately be discarded. 



This reversal of the usual order is attributed to the great delicacy of 

 the young test, to compensate for which a more compact arrangement of 

 the chambers has been acquired. In the later stages of growth, owing to 

 the larger bulk of the protoplasm, the chamber walls can be secreted of 

 such a thickness as to counterbalance the mechanical weakness of their 

 arrangement. 



The contrast in the modes of growth of the megalospheric and micro- 

 spheric forms is similarly explained, the small size of the latter in the 

 early stages of growth calling for an arrangement, which is less urgently 

 needed in the later stages, or by the megalospheric form. In the more 

 perfected genera, however (as Quinqueloculina), the tests of the forms of 

 both generations are moulded on the most compact type. 



Thus Rhumbler, like Carpenter, regards the multiform tests of 

 Foraminifera as of great value in tracing out phylogeny, but for precisely 

 opposite reasons, for while Carpenter considers the early phases as repre- 

 senting a stage through which the stock has passed, Rhumbler sees in 

 them the higher stage towards which it is advancing. 



As will be gathered from what has gone before, it does not appear 

 to me that sufficient reason has been shown for discarding the view of 

 Carpenter. 



Another remarkable phenomenon met with among the Fora- 

 minifera is that of Isomorphism. It may be denned as the occurrence 

 under similar external forms of species belonging to distinct 

 stocks. 



Perhaps the most striking instance of it is presented by the 

 Miliolidea and Nummulitidea. It has been pointed out how in 

 the latter family the series Operculina, Heterostegina, Cydodypeus runs 

 parallel with the Peneroplis, Orbiculina, OrUtolites series of the 

 Miliolidea, and we have seen that in Heterostegina, as well as in 

 Polystomella and other allied genera, the tests are to some degree 

 extended in the spiral axis owing to the equitant character of 

 the chambers. The resemblance between the corresponding terms 

 in the two series is rendered all the more remarkable by considera- 

 tion of the forms included in the genus Fusulina, which, at the 



