606 THE SPIRAL SHELLS [ch. 



But that this, or something of the kind, does actually occur 

 we can scarcely doubt. For example in the genus Penerophs, the 

 first portion of the shell consists of a series of chambers arranged 

 in a spiral or nautiloid series; but as age advances the spiral is 

 apt to be modified in various ways*. Sometimes the successive 

 chambers grow rapidly broader, the whole shell becoming fan- 

 shaped. Sometimes the chambers become narrower, till they no 

 longer enfold the earher chambers but only come in contact each 

 with its immediate predecessor: the result being that the shell 

 straightens out, and (taking into account the earher spiral portion) 

 may be described as crozier-shaped. Between these extremes of 

 shape, and in regard to other variations of thickness or thinness, 

 roughness or smoothness, and so on, there are innumerable 

 gradations passing one into another and intermixed without regard 

 to geographical distribution : — " wherever Penerophdes abound 

 this wide variation exists, and nothing can be more easy than to 

 pick out a number of striking specimens and give to each a dis- 

 tinctive name, but in no other way can they be divided into 

 ' species.^ "f'^ Some writers have wondered at the pecuhar 

 variabihty of this particular shell % ; but for all we know of the 

 hfe-history of the Foraminifera, it may well be that a great 

 number of the other forms which we distinguish as separate species 

 and even genera are no more than temporary manifestations of 

 the same variability § . 



* Cf. Brady, H. B., Chollenger Rep., Foraminifera, 1884, p. 20.3, pi. xm. 



t Brady, op. cit., p. 20G; Batsch, one of the earliest writers on Foraminifera, 

 had already noticed that this whole series of ear-shaped and crozier-shaped shells 

 was filled in by gradational forms; Conchylien des Seesamles, 1791, p. 4, pi. vi, 

 fig. 15 a-f. See also, in particular, Dreyer, Peneroplis; eine Studie zur biologischen 

 Morphologic und zur Speciesfrage, Leipzig, 1898 ; also Eimer und Fickert, Artbildung 

 und Verwandschaft bei den Foraminiferen, Tnbinger tool. Arbeiten, ni, j). .35, 

 1899. 



J Doflein, Profozoenhnide, 1911. p. 263; "Was diese Art veranlasst in dieser 

 Weise gelegentlich zu variiren, ist voi'laufig nocli ganz rathselhaft." 



§ In the case of Globigerina, some fourteen species (out of a very much larger 

 number of described forms) were allowed by Brady (in 1884) to be distinct; and 

 this list has been, I believe, leather added to than diminished. But these so-called 

 species depend for the most part on slight differences of degree, differences in the 

 angle of the spiral, in the ratio of magnitude of the segments, or in their area of 

 contact one with another. Moreover with the exception of one or two "dwarf" 

 forms, said to be limited to Arctic and Antarctic waters, there is no principle of 

 geographical distribution to be discerned amongst them. A species foimd fossil 



