68 CHAS. C. ADAMS. 



Thus certain habitat types have originated many times independ- 

 ently. For example, the fresh water fauna was not formed all 

 at once. This environment has been independently and repeat- 

 edly invaded by very diverse animals and from diverse habitats- 

 The same is equally true of the minor fresh water habitats, such 

 as that of the littoral zone or the rapid water of a brook, etc. 

 The same is equally true of land habitats, such as caves, deserts 

 and many other situations whose biota has been derived from all 

 possible directions. It is thus evident that there are two fairly 

 distinct classes of succession in a given biotic region, the adapta- 

 tional one, in which the ecological aspect is prominent, and the 

 hereditary one, in which the taxonomic or hereditary aspect re- 

 ceives emphasis. 



From the above considerations of habitats and their biota, their 

 successional relations and their convergent habitat and ecological 

 tendencies, we are led to a very natural question : What is their 

 bearing upon migration and dispersal centers ? This relation is 

 very close and unfortunately only too often completely overlooked. 

 If in the study of the life of a given region practical recognition 

 can be made of the above mentioned relations, which are involved 

 in the study of the origin of the biota of given habitats, there will 

 result a very desirable geographic perspective. Such a perspec- 

 tive will greatly aid in the determination of the relative influence 

 of the factors of the environment. As the habitats of many plants 

 and animals change with geographic range it is very desirable to 

 take advantage of this variation in estimating the relative influ- 

 ence of different elements in the environment. In this way we 

 may hope to distinguish between the local and geographic condi- 

 tions, influences which are easily confused. It is primarily their 

 reflex effect upon geographic distribution, and especially upon 

 biotic interpretation which interests us at this time, a subject which 

 can not be separated from a consideration of the relative influence 

 of environmental factors. Habitat studies not only throw impor- 

 tant light upon geographic origin of biotic elements but also upon 

 the conditions of life determining routes of dispersal, and these 

 are often very important elements in biotic interpretation. For 

 it seems at present that it is along this line that we may expect, 

 in the near future, some of the most rapid advances in distribu- 

 tional problems. 



