STANDARD SECTIONS 527 



other words, it never comes back pure. The close student soon learns to 

 detect the several invasions Avith unfailing accuracy. 



Eecurrent Floras unknown 



Eecurrent floras in any way comparable to the above-described recur- 

 rent faunas are not known. Perhaps the nearest approach to this is where 

 certain living plants have partially reoccupied areas from which they were 

 forced by the Pleistocene glaciation, but throughout geologic time they 

 have not "come back" when once excluded from an area. The reason for 

 this is that there is no great, stable, evolutionary cradle— like the ocean — 

 out of which they could migrate and to which they could return on the 

 approach of unfavorable conditions, and from which they could again 

 emerge on restoration of propitious surroundings. It is, of course, true 

 that types of vegetation have continued for varying lengths of time, some 

 even from their earliest known appearance to the living flora, and hardy 

 individual species have often persisted from one epoch into another; but 

 floras once they have disappeared from a region never return to it — they 

 have disappeared forever. This fact undoubtedly adds much to the effi- 

 ciency of plants in geologic correlation. 



Eapidity or plant Migration 



The proposition was long ago advanced that because an identical, or 

 practically identical, flora appeared in two widely separated regions that 

 this was prima facie evidence that it could not be of the same age in the 

 two places, since, it was argued, it must have taken an appreciable length 

 of time to pass from one area to another. This is not believed to be a 

 valid contention, when we take into consideration the known rapidity 

 with which living plants are disseminated. A few examples will serve to 

 make the point plain. The entire vegetation on Krakatau, a volcanic 

 island in the Indian Ocean, 30 miles from the main land, was absolutely 

 destroyed in August, 1883. Within three years it was found that at least 

 20 species, including bacteria, diatoms, blue-green algae, ferns, and flower- 

 ing plants, had gained a new foothold, and within 25 years nearly 150 

 species had reached the island, and the typical tropical jungle was re- 

 stored. The so-called Eussian thistle (Salsola Tragus) was introduced 

 into the Dakotas about 1883, and within 10 years had spread over prac- 

 tically all the Eocky Mountain States. 



A striking example of the migration of a living shell has been worked 

 out by Ulrich. Littorina littoria is known to have spread along the At- 



