GEOGRAPHICAL DISTRIBUTION. 



207 



probable, tliougli perhaps less so than is generally supposed : 

 for the fact is that the former are more accessible to obser- 

 vation than the latter, and are, therefore, more generally and 

 more readily comprehended and acknowledged. Tims, not 

 only because many of the most characteristic forms of the 

 marine and freshwater series are found, on taking a broader 

 view of the subject, to be sufficiently distinct, but also be- 

 cause their distribution depends on causes different in their 

 nature as well as the degree of their action, it will be 

 necessary to treat of the two series separately. Whether the 

 oceanic areas correspond in any way to the terrestrial will 

 be seen in the sequel. 



Fig. 103.— Ganoid scales of Tetragonolepis. 



