GROUPS OF ALLIED SPECIES. 337 



and Triassic forms are now commonly admitted to be 

 teieostean; and even some palseozoic forms have thus been 

 classed by one high authority. If the teleosteans liud 

 really 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 ])een 

 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 any 

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

 running through Pictet's Palaeontology 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 teieostean fishes might formerly have had a 

 similarly confined range, and after having been largely 

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

 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 inclosed basin, in 

 which any great group of marine animals might be multi- 

 plied; 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 

 palseontological 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 foims 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 KNOWN FOSSILIFEKOUS STRATA. 



There is another and allied difficulty, which is much 

 more serious. I allude to the manner in which species 



