60 INVASION 



in climate become sufficiently modified to permit the inva- 

 sion of mesophytes. On the other hand, a meadow ceases 

 to be a barrier to prairie xerophytes during a period of 

 unusually dry years. A peculiar example of the modifica- 

 tion of a barrier is afforded by the defoliation of aspen 

 forests in the mountains, as a result of which poophytes 

 have been enabled to invade them. Nearly all xerophytio 

 stretches of sand and gravel, dunes, blow-outs, gravel-slides, 

 etc, , and even prairies to a certain degree, exhibit a recur- 

 rent seasonal change in spring, as a result of which the hot 

 dry surface becomes sufficiently moist to permit the germi- 

 nation and growth of invaders, which are entirely barred out 

 during the remainder of the year. In an absolute sense, no 

 barrier is complete, since the coldest as well as the dryest 

 portions of the earth's surface are capable, at times at least, 

 of supporting the lowest types of vegetation. Eelatively, 

 however, in connection with the natural spread of terrestrial 

 plants, it is possible to distinguish partial barriers from 

 complete ones. Such a distinction is of importance in the 

 consideration of invasions from a definite region, as it is 

 only in this restricted sense that complete barriers have pro- 

 duced endemism. 



Distance, though hardly to be considered a barrier in the 

 strict sense of the word, unquestionably plays an important 

 part in determining the amount of invasion. The effect of 

 distance is best seen in the case of migration, as it influences 

 ecesis only in those rare cases where viability is affected. 

 The importance of distance, or to take the converse, of prox- 

 imity, is readily ascertained by the study of any succession 

 from denudation. It has been established that the contig- 

 uous vegetation furnishes 75-90% of the constituent species 

 of the initial formation, and in mountainous regions, where 

 ruderal plants are extremely rare, the percentage is even 

 higher. The reason for this is to be found not only in the 

 fact that the adjacent species have a much shorter distance 

 to go and hence will be carried in much greater quantity, but 

 also in that the species of the formations beyond must pass 



