THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 37 



Migration in plants is almost entirely passive; the seeds are carried by 

 purely external agencies for a greater or less distance, and while the move- 

 ment may be rapid in some cases, as with animals, it is apt to be very slow. 

 Seward estimates the average amount of movement in forests by self- 

 seeding as a yard a year, an amount that is practically negligible. 



(e) AUTOCHTHONY (ORIGINATING IN PlACE). 



The assumption that particular spots are the original home of certain 

 forms and that they have migrated in certain definite directions has been 

 made in a large number of cases. This assumption has, of course, placed 

 the original home of any form or group at the locality where it is found lowest 

 in the geological series and has traced the migrations by its appearance at 

 higher levels in successive spots, but the method is open to objection in 

 many regards. If Ulrich's assumption that evolution of the invertebrate 

 forms has taken place in the ocean basins is correct, the first appearance 

 in epicontinental sea deposits is to some extent accidental and the statement 

 of direct migration is only, after all, a statement of where we know the fauna 

 to occur at later dates. Such conclusions should be most tentatively stated. 

 Indeed, if Ulrich is correct, autochthony in observable regions would be 

 very rare. 



(/) Accidental Introduction. 



By accidental introduction is meant the sporadic dispersal of indi- 

 viduals as opposed to the migration of an entire or a large portion of a 

 fauna or flora. Such sporadic inclusions of unexpected elements may be 

 the result of transmission of the living individuals or of the body after death. 

 In the second case the condition will be revealed by the fact of the presence 

 of but a few individuals or a single specimen and may be dismissed as 

 accidental. Herein lies a grave danger. We have seen how bodies of plant 

 material may be swept by normal streams, by floods, by wind storms, by 

 high tides from one locality to another, and the remains preserved far from 

 their natural habitat. If such occurrences are carelessly treated they may 

 involve some erroneous conclusions of the first magnitude, especially if the 

 observer is inclined to give heed to the presence of unique and peculiar forms 

 in correlation rather than to the "matching of species." Premature assump- 

 tions regarding the removal of barriers or of migrations may easily arise. 



In the first case, living forms may be carried by such accidents and 

 find a favorable environment in which they will live and multiply to a 

 remarkable extent. Cases of this form of dispersal across barriers and 

 speculations as to other possibilities have multiplied to large numbers and 

 it is unnecessary to repeat them.^ It may be recalled, as instances in point, 

 that eggs and seeds are carried in the mud attached to the feet of aquatic 



^ Matthew, W. D., Climate and Evolution, Annals of N. Y. Acad. Sci., vol. xxiv, especially 

 pp. 200-204, 1915- 



