Chap. X.] GROUPS OF ALLIED SPECIES. 285 



have abundant evidence of the existence of this group of animals 

 during the secondary period. 



The case most frequently insisted on by paleontologists of the 

 apparently sudden appearance of a whole group of species, is that of 

 the teleostean fishes, low down, according to Agassiz, in the Chalk 

 period. This group includes the large majority of existing species. 

 But certain Jurassic and Triassic forms are now commonly admitted 

 to be teleostean ; and even some palaeozoic forms have thus bwn 

 classed by one high authority. If the teleosteans had ready 

 appeared suddenly in the northern hemisphere at the commencement 

 of the chalk formation, the fact would have been highly remarkable; 

 but it would not have formed an insuperable difficulty, unless it 

 could likewise have been shown that at the same period the species 

 were suddenly and simultaneously developed in other quarters of 

 the world. It is almost superfluous to remark that hardly auy 

 fossil-fish are known from south of the equator ; and by running 

 through Pictet's Paleontology it will be seen that very few species 

 are known from several formations in Europe. Some few families of 

 fish now have a confined range ; the teleostean fishes might formerly 

 have had a similarly confined range, and after having been largely 

 developed in some one sea, have spread widely. Nor Lave we 

 any right to suppose that the seas of the world have always been 

 so freely open from south to north as they are at present. Even at 

 this day, if the Malay Archipelago were converted into land, the 

 tropical parts of the Indian Ocean would form a large and perfectly 

 enclosed basin, in which any great group of marine animals might 

 be multiplied; and here they would remain confined, until some of 

 the species became adapted to a cooler climate, and were enabled to 

 double the Southern capes of Africa or Australia, and thus reach 

 other and distant seas. 



From these considerations, from our ignorance of the geology of 

 other countries beyond the confines of Europe and the United 

 States, and from the revolution in our paleontological knowledge 

 effected by the discoveries of the last dozen years, it seems to me 

 to be about as rash to dogmatize on the succession of organic forms 

 throughout the world, as it would be for a naturalist to land for five 

 minutes on a barren point in Australia, and then to discuss the 

 number and range of its productions. 



On the sudden Appearance of Groups of allied Species in the 

 lowest knoiun Fossiliferous Strata. 

 There is another and allied difficulty, which is much more serious. 

 I allude to the manner in which species belonging to several of the 



