166 OUSTERS OF THE CHALK, 



what may be called the direct action of c natural 

 selection,' since by the well-established principle of 

 ' correlation' the variation in one part of an organism is 

 nearly or quite certain to produce variations in other 

 parts. ' If any such change did occur,' it is argued, ' it 

 must have been per saltum> since with these mollusks, 

 numerous as they are, there are no forms that can 

 fairly be recognised as transitional.' But this appeal 

 to the evidence of facts is somewhat premature. The 

 immense difference pointed out between the geological 

 records of England and France in regard to these very 

 oysters of the chalk, leaves it perfectly open for us to 

 suppose that even the comparatively full French record 

 is itself exceedingly imperfect, and that the transitional 

 forms have either not been preserved, or remain yet to 

 be discovered. Mr. Darwin gives reasons for believing 

 that when variation once begins it continues with some 

 vigour ; hence, between two settled wide-spread species 

 connected genealogically together we might expect a 

 large number of transitional varieties, each represented 

 by only a few individuals, so that the whole number of 

 these transitional forms might well be lost to the 

 genealogical record. 



Finally, the objection from the scarcity of oysters at 

 the present day, compared with the great abundance of 

 species in the past, does not really touch the theory of 

 development, which is concerned to explain how species 

 come into existence, not how they go out of it. That 

 varieties, species, genera, have been superseded or ex- 

 tinguished, within longer or shorter periods, is a fact 



