608 THE SPIRAL SHELLS ' [ch. 



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 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 

 structi^ral 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 Rhabdammina 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 

 Foraminifera, Dreyer and Rhumbler (following Neumayr) recog- 

 nise 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 perpetua- 

 tion; 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 prim- 

 eval seas; and finally, the gradual development and increasing 



* Dreyer, F., Principien der Geriistbildung bei Rhizojioden, etc., Je7i. Zcihchr. 

 xxvT, pp. 204-468, 1892. 



