238 



EXPLANATIONS. 



conditions, or resulting from equality of grade in the 

 scale. True affinities — and these are the affinities of 

 genealogy — are not to be looked for horizontally amongst 

 ordtrs, 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 charac- 

 ters of the class next below. Thus, the perennibranchiate 

 batrachia in their order, the ichthyosauri in the series of 

 crocodilia, and the divers among birds, all exhibit an 

 affinity to fish. The cetacea and phocidse, which I re- 

 gard as the immediate basis of the pachydermata, carniv- 

 ora, and other orders of terrestial mammals, ought, ac- 

 cording to this view, to show an alliance to the reptiles ; 

 and such a connection does exist between the cetacea and 

 certain marine sauria , but from the general extinction 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 outline of the pro- 

 gress of the animal kingdom is here designed. Excep- 

 tions as to the course which development has taken ap- 

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

 grades of organization are not determinate in this respect, 

 but may be reached by steps of unequal length. Thus 

 for example, the marsupials appear very clearly a de- 

 velopment from certain birds ; probably the rodent and 

 edentate orders are derived through the same channel. 

 From the approach made by certain of the reptilia to 

 birds, we may surmise that there also there are excep- 

 tions to the rule. In short, the progress of animality m 

 the different stirpes has been attended by peculiarities 

 which evidently affix peculiar characters to each, and 

 make the idea of a difference in time not only probable, 

 but unavoidable. 



Regarding the animal kingdom simply as a combination 

 of independent stirpes, each with its distinct affinities, 

 the theory of transmutation puts on a totally new aspect ; 

 so truly is this the case, that transmutation is hardly any 

 longer a term appropriate to the idea. The difficulty ol 

 supposing such changes as that from the rodent to the ru- 

 minant, or the carnivorous animal to the quadrumane, 

 vanishes, leaving only transitions from one form to 

 another of a series generally similar — from the aquatic 



