INDIRECT EVIDENCE. 47 



animals died, their shells sank to the bottom of the 

 lake and there accumulated for ages. The shells 

 are now found in this old lake-bottom in great 

 quantities, and being found as they are in layers, 

 the whole history of the animals can be studied by 

 examining successive layers. And most beautifully 

 has such study shown the origin of species. Begin- 

 ning with a few sample varieties, there is a rapid 

 divergence until very diverse forms are reached, so 

 diverse, indeed, that they must unhesitatingly be set 

 down as different species. At least this would be 

 the fact, were it not that all these diverse forms are 

 connected by an infinite number of intermediate 

 forms. It is of course possible to claim here, as 

 elsewhere, that we are simply dealing with a single 

 highly variable species. But the extreme forms are 

 so very different from each other that this claim is a 

 very weak one. If species be defined as forms not 

 connected with each other by intermediate grada- 

 tions, of course nothing is to be said. But if we 

 use any other definition for species that it is possi- 

 ble to apply, the Steinheim Lake appears as the 

 site of the origin of a number of species from a 

 common ancestor. 



One very interesting argument for the common 

 origin of the species of any genus, is found in a class 

 of variations called reversions. Animals and plants 

 frequently show quite suddenly features which, upon 

 close examination, are found to be the characteris- 

 tics of their ancestors characteristics long since 

 lost but suddenly reappearing. The circumstances 

 which bring about these reversions are quite varied. 



