THE FORAMINIFERA 135 



in some, as in Peneroplis, the size of the megalosphere may, in 

 exceptional cases, fall below that of the microsphere. In this 

 genus, as we have seen, the microspheric form is also to be dis- 

 tinguished by the absence of the spiral passage. In Discorbina and 

 Truncatulina there is no such structural feature to distinguish the 

 two forms, nor are they always to be recognised by the size of 

 the central chambers. There is reason to believe, however, that 

 they differ in nuclear characters, and mode of reproduction. 

 Whether or not the two modes of reproduction prevail through- 

 out the simpler forms of Foraminifera cannot at present be 

 stated. 



The Multiform Condition. The significance of this condition is 

 one of the most interesting problems presented by the Foraminifera. 

 Perhaps the simplest case of its occurrence is that of Polytrema 

 (p. 120). We have seen that in the earliest stages of life this 

 organism is free, and secretes a test which resembles in many of 

 its features that typical of the Rotalidae. After it has become 

 adherent the rotaline mode of growth is exchanged for one 

 adapted to the attached habit, and the test assumes an encrusting 

 or arborescent form. 



In the case of Polytrema, then, it seems clear that the arrange- 

 ment of the chambers formed early in life repeats that of the 

 rotaline stock from Avhich it sprang, while the later chambers are 

 disposed on a plan acquired as it has diverged from that 

 stock. 



Again, the more complex members (Orbitolites and Cydodypeus) 

 of the Peneroplis-Orbitolites and Operculina-Cydodypeus series present 

 excellent examples of the multiform condition. The facts that 

 each of these is a series of closely related genera, and that the 

 simpler members of each present in a permanent form the arrange- 

 ment which is transitory in the growth of the more complex, 

 appear to give substantial support to the view urged by Car- 

 penter that the stages which we have called peneropline and 

 orbiculine, operculine and heterostegine, in the growth of Orbitolites 

 and Cydodypeus respectively, are, in fact, repetitions in ontogeny 

 of a phylogenetic history. 



The application of this explanation to the multiform Miliolinidae 

 appears less satisfactory because the earlier (quinqueloculine) plan 

 of growth is somewhat more complex than the later, and we should 

 not therefore expect it to be the more primitive. We need not 

 assume, however, that the course of development has always been 

 in the direction from simple to complex. 



Closely connected with this question is the fact that the multi- 

 form condition is, as we have seen, much more pronounced in the 

 microspheric than in the megalospheric form of a species. In a 

 former paper I suggested (21) that a partial explanation of the 



