870 THE SPIRAL SHELLS [ch. 



evolutionist — to whom evolution was a mental concept, involving 

 order and continuity in thought but not an actual sequence of events 

 in time. Such a conception of evolution is not easy for the modern 

 biologist to grasp, and is harder still to appreciate. And so it is 

 that even those who, like Dreyer* and like Rhumbler, study the 

 foraminiferal shell as a physical system, who recognise that its 

 whole plan and mode of growth is closely akin to the phenomena 

 exhibited by fluid drops under particular conditions, and who 

 explain the conformation of the shell by help of the same physical 

 principles and mathematical laws^yet all the while abate no jot 

 or tittle of the ordinary postulates of modern biology, nor doubt 

 the validity and universal applicability.of the concepts of Darwinian 

 evolution. For these writers the biogenetisches Grundgesetz remains 

 impregnable. The Foraminifera remain for them a great family 

 tree, whose actual pedigree is traceable to the remotest ages; in 

 which historical evolution has coincided with progressive change; 

 and in which structural fitness for a particular function (or functions) 

 has exercised its selective action and ensured "the survival of the 

 fittest." By successive stages of historic evolution we are supposed 

 to pass from the irregular Astrorhiza to a Rhahdammina with its 

 more concentrated disc; to the forms of the same genus which 

 consist of but a single tube with central chamber; to those where 

 this chamber is more and more distinctly segmented; so to the 

 typical many-chambered Nodosariae; and from these, by another 

 definite advance and later evolution to the spiral Trochamminae. 

 After this fashion, throughout the whole varied series of the Fora- 

 minifera, Dreyer and Rhumbler (following Neumayr) recognise so 

 many successions of related forms, one passing into another and 

 standing towards it in a definite relationship of ancestry or descent. 

 Each evolution of form, from simpler to more complex, is deemed 

 to have been attended by an advantage to the organism, an 

 enhancement of its chances of survival or perpetuation ; hence the 

 historically older forms are on the whole structurally the simpler; 

 or conversely, the simpler forms, such as the simple sphere, were 

 the first to come into being in primeval seas; and finally, the 

 gradual development and increasing comphcation of the individual 



* F. Dreyer, Prinzipien der Gerustbildung bei Rhizopoden, etc., Jen. Zeitschr. 

 XXVI, pp. 204-468, 1892. 



