72 WATER REPTILES OF THE PAST AND PRESENT 



resemblances in unrelated animals are known as parallel or con- 

 vergent evolution. It has often been difficult to distinguish 

 between convergent evolution and direct evolution, and difiiculties 

 still perplex and trouble the student of natural history in every 

 branch of life. Not till all such problems are solved can we hope 

 to attain the true classification of animals and plants. The whales 

 a century ago were considered merely breathing fishes; the ichthyo- 

 saurs until a quarter of a century ago were supposed to be the 

 direct descendants of fishes; lizards and crocodiles were grouped 

 together in a single order; and salamanders were called reptiles 

 not very long ago. 



Perhaps the reader will be able from the foregoing to under- 

 stand and appreciate better some of the difficulties that confront 

 the paleontologist in his attempts to solve the problems of past life; 

 to understand why he sometimes makes mistakes, for he has by 

 no means yet learned all the permutations of the skeleton in any 

 class of vertebrates, and is not sure that the laws he accepts are 

 not subject to modifications and exceptions. If he is truly scien- 

 tific he hesitates long in prophesying or conjecturing. 



