84 MODERN IDEAS OP EVOLUTION 



that the geological record does not justify us in 

 accepting any of the received theories of descent with 

 modification. The subject is too large for discussion 

 here, but a single illustration from a very familiar 

 animal may show the results to which it leads when 

 we follow the actual guidance of facts, without inter 

 calating, as is the wont of evolutionists, a constant 

 succession of suppositions. 



The oyster belongs to the great and ubiquitous 

 class of the bivalve molluscs, and we know representa 

 tives of the genus all the way from the Carboniferous 

 to the Modern time, while we know very well the 

 changes of the individual animal from the egg to the 

 perfect form. 1 The oyster begins life as a free- 

 moving creature, without shell, and with those curious 

 movable threads called cilia, by means of which so 

 many humble animalculae move in the water. In 

 this state it shows little evidence of its future de 

 velopment. When it first assumes a shell, this has 

 already two valves placed on the right and left sides of 

 the animal, but quite different from those of the adult. 

 They are nearly circular, smooth, and marked with 

 regular concentric lines of growth. This is their 

 condition when about a tenth of an inch in diameter. 

 At this stage they resemble the valves of a cockle or 

 a venus shell much more than those of an oyster. 

 Another curious point here is that, while the oyster 



1 Jackson, Development of the Oyster, Proc. Boston Soc. Nat. 

 Hist. 1 888. 



