692 JAMES P ERR IN SMITH 



glaciation in this region near the beginning of Permian time 

 killed off the Paleozoic flora and allowed the Glossopteris flora to 

 get a foothold earlier than was the case where there was no 

 glaciation. Such phenomena approach the nature of catastro- 

 phes, and show that Cuvier's doctrine was not altogether wrong 

 after all, and he probably had something like this in mind, 

 although not formulated. These facts, too, show that the prin- 

 ciples on which Barrande's 1 doctrine of "colonies" was founded 

 were right, even though it has since been found that the particu- 

 lar cases on which he based his colonies were only younger rocks 

 carried into the midst of older beds by dislocations, Barrande's 

 idea seems to have been that in certain separated basins a new 

 type of life would be introduced before it appeared elsewhere, 

 and that by changes in physical geography these precursor 

 faunas would be intercalated with those of older type. The 

 modern doctrine of colonies, on the other hand, is that older 

 faunas have often been preserved in places where no great 

 changes have taken place in the conditions necessary for their 

 'ife, and that these older surviving faunas have been mingled as 

 anachronisms with the younger through immigration made pos- 

 sible by the removal of barriers, or changes in the direction of 

 ocean currents. 



Synchronism vs. homotaxis. — Forms are said to be heterochronous 

 when they occur at different horizons, in different regions. Now it 

 is possible that the same species seldom occurs at exactly the same 

 time in two widely separated places ; it must originate at the one 

 and migrate toward the other, or migrate to both from a third 

 place. This would take time, and it is supposable that the species 

 might be entirely extinct at the point of origin before it reaches its 

 second habitat, or become so greatly modified on its journey 

 as to require a new name, or a number of new names. A case 

 in point is the migration and development of Ceratites nodosus in 

 the middle Trias of Germany. In the North-German basin this 

 species is exceedingly common in the Muschelkalk, and exceed- 

 ingly variable, but the boldest species-maker has not yet dared 



1 Systeme Silur. du Centre de la Boheme, Vol. I, p. 73 et seq. 



