CHAP. X.] GROUPS OF ALLIED SPECIES. 267 



tertiary series. This was a sore trouble to me, adding as I then 

 thought one more instance of the abrupt appearance of a great 

 group of species. But my work had hardly been published, when 

 a skilful palaeontologist, M. Bosquet, sent me a drawing of a per- 

 fect specimen of an unmistakeable sessile cirripede, which he had 

 himself extracted from the chalk of Belgium. And, as if to make 

 the case as striking as possible, this cirripede was a Chthamalus, a 

 very common, large, and ubiquitous genus, of which not one species 

 has as yet been found even in any tertiary stratum. Still more 

 recently, a Pyrgoma, a member of a distinct sub-family of sessile 

 cirripedes, has been discovered by Mr. Woodward in the upper 

 chalk ; so that we now have abundant evidence of the existence 

 of this group of animals during the secondary period. 



The case most frequently insisted on by palaeontologists 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 been classed by one high authority. If the teleosteans had 

 really appeared suddenly in the northern hemisphere at the com- 

 mencement 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 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 teleostean 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 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 palaaontological knowledge 

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



10 



