172 CLASSIFICATION OF SERES. 



erosion generally is destructive to vegetation, or at least retrogressive, while 

 the influence of deposition is constructive or progressive. (170) 



"Of less interest, perhaps, to the physiographer than are the vegetative 

 changes hitherto considered, but of far greater import to the plant geographer, 

 are the vegetative changes that are due to plant and animal agencies. These 

 are found to have an influence that is more diversified than is the case with 

 physiographic agencies; furthermore, their influence can be more exactly 

 studied, since they are somewhat readily amenable to experimental control, 

 but particularly because they operate with sufficient rapidity to be investigated 

 with some exactness within the range of an ordinary lifetime. If, in their 

 operation, regional agencies are matters of eons, and topographic agencies 

 matters of centuries, biotic agencies may be expressed in terms of decades. (171) 



"At first thought, it seems somewhat striking that far-reaching vegetative 

 changes take place without any obvious climatic change and without any 

 marked activity on the part of ordinary erosive factors. Indeed, it is probably 

 true that the character of the present vegetative covering is due far more to 

 the influence of biotic factors than to the more obvious factors previously 

 considered. So rapid is the action of biotic factors that not only the climate, 

 but even the topography may be regarded as static over large areas for a con- 

 siderable length of time. It has been said that many of our Pleistocene 

 deposits exhibit almost the identical form which characterized them at the 

 time of their deposition, in other words, the influence of thousands of years of 

 weathering has been insufficient to cause them to lose their original appearance. 

 These thousands of years would have sufficed for dozens and perhaps for 

 himdreds of biotic vegetative cycles. Many a sand dune on the shores of 

 Lake Michigan is clothed with the culminating mesophytic forests of the eastern 

 United States, and yet the sand dunes are products of the present epoch; 

 furthermore, sand is regarded generally as a poor type of soil in which to 

 observe rapid succession. If a clay upland were denuded of its forest and its 

 humus, it is believed that only a few centuries would suffice for the mesophytic 

 forest to return. (172) 



"Although they grade into one another as do all phenomena of nature, we 

 may recognize climatic agencies, which institute vegetative cycles whose 

 duration is so long that the stages in succession are revealed only by a study 

 of the record of the rocks. Within one climatic cycle there may be many 

 cycles of erosion, each with its vegetative cycle. The trend of such a cycle can 

 be seen by a study of erosive processes as they are taking place to-day, but 

 the duration of the cycle is so long that its stages can be understood only by 

 a comparison of one district with another; by visiting the parts of a river 

 from its source to its mouth, we can imagine what its history at a given point 

 has been or is to be. Within a cycle of erosion there may be many vegetative 

 cycles, and among these there are some whose duration is so short that exact 

 study year by year at a given point makes it possible to determine not only 

 the trend of succession, but the exact way in which it comes about. It is 

 clear therefore that vegetative cycles are not of equal value. Each climatic 

 cycle has its vegetative cycle; each erosive cycle within the climatic cycle in 

 turn has its vegetative cycle; and biotic factors institute other cycles, quite 

 independently of climatic or topographic changes." (181) 



In the last two statements Cowles has made evident one of the chief 

 objections to a primary classification of successions as regional, topographic, 

 and biotic. This is that these successions actually represent three totally 

 different degrees of development or developmental sequences. His biotic 

 succession is a developmental unit, a unit succession or sere; the topographic 



