FISHES 153 



Hence in the comparatively brief period geologically 

 speaking since the appearance of the fishes, no 

 species could be developed from them with a 

 fundamentally different structure from their ancestors. 



There are no organisms that descend from the verte- 

 brates, and differ from them so widely, as the fishes 

 do from the insects ; the chief reason is, that there 

 has not been sufficient time for such a transformation. 



We can understand, therefore, why the structure of 

 all vertebrates is essentially the same. All of them 

 have an internal skeleton, and have the organs of 

 nutrition and reproduction on the ventral side, and 

 the nervous system on the dorsal side. But the 

 similarity goes still further, and this also is clearly 

 the effect of natural selection. Selection always 

 builds on a given material ; it makes use of actual 

 adaptations, preserves them when change is un- 

 necessary, and modifies them when some new adapta- 

 tion of the species requires it. If, for instance, there 

 was already amongst the fishes a skull with the 

 function of protecting the brain, there was no need 

 for the formation of a new structure to contain the 

 ever-growing brain in the classes that developed from 

 the fishes. Therefore, the bones of the fish's skull 

 were retained in the new classes, and were enlarged 

 and modified according to need. As a fact, even the 

 human skull is generally composed of the same bony 

 plates that we find in the fishes. It is well known that 

 students used to be puzzled by the fact of the upper 

 jaw of all vertebrates containing four pieces of bone, 

 while in man it had only two, until Goethe discovered 



