134 THE FORAMINIFERA 



exactness. This view was insisted on by Carpenter, who, in the 

 " Challenger " Report on Orbitolites (p. 9), quotes with approval the 

 doctrine that among the porcellanous and vitreous Foraminifera 

 " everything passes into everything else." 



Carpenter, indeed, held (I.e. p. 8) that "the ordinary notion of 

 species as assemblages of individuals marked out from each other 

 by definite characters that have been genetically transmitted from 

 original prototypes similarly distinguished, is quite inapplicable to 

 the group of the Foraminifera." And again, in the Introduction we 

 read (8, p. 56): "The impracticability of applying the ordinary 

 method of definition to the genera of the Foraminifera becomes 

 an absolute impossibility in regard to species. For whether or 

 not there really exist in this group generic assemblages capable 

 of being strictly limited by well-marked boundaries, it may be 

 affirmed with certainty that among the forms of which such 

 assemblages are composed, it is the exception, not the rule, to 

 find one which is so isolated from the rest by any constant and 

 definite peculiarity, as to have the least claim to rank as a natural 

 species." 



The question, however, appears to be not whether all inter- 

 mediate terms do or do not exist between dissimilar forms, but 

 whether the whole body of forms, as they occur in nature, tend to 

 group themselves, or are aggregated about certain centres. If this 

 is the fact, and the forms, as they occur in nature, are disposed not 

 in a continuous series, but in a discontinuous one, the large number 

 of individuals being grouped about distinct centres, we have the 

 phenomenon which is comparable with that of species in other 

 animals and in plants, whether the centres are or are not connected 

 by intermediate terms. To refuse to recognise the existence of these 

 centres, because transitional forms exist between them, is to ignore 

 an essential fact. 



In a very large number of cases, at any rate, such centres do 

 exist among the Foraminifera, as among other organised beings, 

 and the characters of the middle individuals of them are those of 

 the species. 



The dimorphism of Foraminifera depends, as we have seen, on 

 two modes of reproduction, which recur in a cycle of generations. 

 The megalospheric form arises by the multiple fission of a single 

 parent, while there are strong grounds for concluding that the 

 microspheric form arises from a zygote, formed by the conjugation 

 of zoospores. 



The phenomena of dimorphism are exhibited in the size of the 

 initial chambers, in the nuclear characters, in the mode of reproduc- 

 tion, and, often, in the plan of growth. In most of the species of 

 Foraminifera in which we have evidence of the sizes of the initial 

 chambers, they are strongly contrasted in the two forms, although 



