86 MODERN IDEAS OF EVOLUTION 



plaited and unequally valved condition of the 

 adult. 



Hence we might make such statements as that 

 the oyster was originally a monomyarian with anterior 

 adductor ; but no such mollusc is known in an adult 

 state ; then it was a dimyarian with smooth equivalve 

 shell, and of this form are many adult bivalves, both 

 ancient and modern. This is the history of the in 

 dividuals ; but have we any evidence that it is the 

 history of oysters in geological time ? We know 

 fossil oysters of the ordinary style, though small, as 

 far back as the Carboniferous age, but we know no 

 earlier bivalves having precisely the properties of the 

 early stage. So, though the young spat of these 

 primitive oysters may have been like that of the 

 modern ones, we cannot believe that it came from 

 the eggs of any species known earlier. Still this is 

 possible. Some bivalve of the pre-Carboniferous 

 or Carboniferous age, a Pterinea for example, may 

 have produced eggs which, when hatched, attached 

 themselves, and, unlike their parents, produced 

 irregular one-sided shells like the oyster, and their 

 progeny may have continued to do the same. If so, 

 they showed a miraculous persistency in this course 

 of degradation ; and not only so, but in pretty early 

 times, the Jurassic age for example, they had plaited 

 themselves up to an extreme degree of plication and 

 irregularity not surpassed in any subsequent time. 

 Since the Carboniferous, when two so-called species 



