DEVELOPMENT OF THE ANIMAL KINGDOM. 73 



resulting from equality of grade in the scale. 

 True affinities — and these are the affinities of 

 genealog}^ — are not to be looked for horizontally 

 amongst orders, but vertically, from an order in 

 one class to the corresponding order in the class 

 next higher. Generally, the first and lowest 

 forms of the orders in a class are marine, and 

 often these are of comparatively large size. We 

 usually see in them a vestige of the essential cha- 

 racters of the class next below. Thus, the peren- 

 nibranchiate batrachia in their order, and the 

 ichthyosauri in the series of crocodilia, exhibit 

 an affinity to fish. The cetacea and phocidae, 

 which I regard as the immediate basis of the 

 pachydermata, camivora, and other orders of ter- 

 restrial mammals, ought, according to this view, 

 to show an alliance to the reptiles ; and such a 

 connexion does exist between the cetacea and 

 certain marine sauria ; but from the general ex- 

 tinction of the marine reptiles, the linking of the 

 mammals to that lower class is less clearly seen 

 than might be wished. It must be kept in 

 view that only an outUne of the progress of the 

 animal kingdom is here designed. Exceptions as 

 to the course which development has taken appear 

 to be by no means few ; leading to the idea that 

 e 



