REPTILES AND AMPHIBIANS 125 



into the hands of experts, we should not be surprised 

 that we have not fossil remains of all the transitional 

 types. We must not expect to find the remains of long 

 series of ancestors of any particular species, showing us 

 how it was gradually converted into a different one. 

 We must not question the theory of evolution because no 

 human skeletons have been preserved in which the 

 non-human element predominates. 



If, then, such discoveries are made, if the famous 

 Steinheim snails 1 bring before us the conversion of one 

 species into another in all its stages, and if the 

 Neanderthal skull and other human remains give us 

 information as to a primitive humanity, we must learn 

 to appreciate the fortunate accident to which we owe the 

 preservation. As a matter of fact, however, we shall 

 claim full credit for the theory of evolution precisely 

 because the evidence of geology, in spite of its 

 incompleteness, affords striking testimony to the truth 

 of it. 



We regard the remains of those monstrous reptiles 

 with astonishment, and ask how it was possible for 

 such powerful creatures to become extinct. We find, 

 however, that towards the close of the Cretaceous 

 period gigantic sharks appeared, measuring twenty- 

 eight yards in length, and we can well believe that 

 they finished off a good many of the ichthyosauri. But 



1 These snails are found in immense numbers in the Steinheim 

 deposits. One stem-form has divided into four groups of varieties, and 

 the transitional forms have been admirably preserved. The stem-form 

 is lowest in the strata, the transitional forms higher up in proportion to 

 their divergence from it. 



