XI] OF BIVALVE SHELLS 831 



one of those cases in which not only does the form of the shell 

 vary, but geologists recognise, now and then, a trend, or progressive 

 sequence of variation, from one stratum or one "horizon" to 

 anotlier. In short, as time goes on, we seem to see the shell growing 

 thicker or widey, or more and more spirally curved, before our 

 eyes. What meaning shall we give, what importance should we 

 assign, to these changes, and what sort or grade of evolution do 

 they imply? Some hold that these palaeontological features are 

 "strictly comparable with those on which the geneticist bases his 

 factorial studies"; and that as such they may shew "linkage of 

 characters," as when "in the evolution of Gryjphaea the area of 

 attachment retrogresses as the arching progresses"*. These are 

 debatable matters. But in so far as the changes depend on mere 

 gradations of magnitude, they lead indeed to variety but fall short 

 of the full concept of evolution. For to quote Aristotle once again 

 (though we need not go to Aristotle to learn it) : "some things shew 

 increase but suffer no alteration ; because increase is one thing and 

 alteration is another." 



The so-called "spiral arms" of Spirifer and many other Brachio- 

 pods are not difficult to explain. They begin as a single structure, 

 in the form of a loop of shelly substance, 

 attached to the dorsal valve of the shell, 

 in the neighbourhood of the hinge, and 

 forming a skeletal support for two cihate 

 and tentaculate arms. These grow to a 

 considerable length, coiling up within the 

 shell that they may do so. In Terehratula 

 the loop reilaains short and simple, and is 

 merely flattened and distorted somewhat 

 by the restraining pressure of the ventral 

 valve ; but in Spirifer, Atrypa, Athyris and 



•x / ^^^'^ Fig. 406. Skeletal loop of Terg- 



many more it forms a watchsprmg coil on j,^^^^ p^^^ ^^^^^^ 

 either side, corresponding to the close- 

 coiled arms of which it was the support and skeleton. In these 

 curious and characteristic structures we see no sign of progressive 



* H. H. Swinnerton, Unit characters in fossils, Biol. Reviews, vii, pp. 321-335, 

 1932; of. A. E. Truman, Oeol. Mag. lix, p. 258, lxi, p. 358, 1922-24. 



