868 THE SPIRAL SHELLS [ch. 



to turn one so-called species into another among the Foraminifera. 

 To what extent this process actually occurs, we do not know. 



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

 can scarcely doubt. For example in the genus Peneroplis, 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. Some- 

 times the chambers become narrower, till they no longer enfold the 

 earlier chambers but only come- in contact each with its immediate 

 predecessor: the result being that the shell straightens out, and 

 (taking into account the earlier 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 Peneroplides 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 distinctive name, but in no other way can they be 

 divided into ' species.' "f Some writers have wondered at the 

 peculiar variability of this particular shellj; but for all we know 

 of the life-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 variabiUty§. 



* Cf. H. B. Brady, Challenger Rep., Foraminifera, 1884, p. 203, pi. xin. 



t Brady, op. cit. p. 206; 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 Seesandes, 1791, p. 4, pi. vi, 

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

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

 und Verwandschaft bei den Foraminiferen, Tuhinger zool. Arbeiten, ni, p. 35, 

 1899. 



X Doflein, Protozoenlcunde, 1911, p. 263: "Was diese Art veranlasst in dieser 

 Weise gelegentlich zu variiren, ist vorlaufig noch 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, rather 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" 



