PLANTS O'F SOUTHERN NEW JERSEY. 103 



south of the true habitat of their species, and the immediate spot 

 may be such as to enable the young plant to persist for a few 

 years or a few generations, though it eventually perishes. So, 

 too, when a species of plant is practically exterminated, local 

 colonies will persist in spots where the immediate environment 

 is suitable for their existence, while similar colonies of mammals 

 require a very much larger area of congenial environment to 

 prevent extermination. 



Two lines of investigation are often confused in the study of 

 geographic distribution : i. e. ( i ) the study of present day distri- 

 bution and the mapping of existing life zones and life areas, and 

 (2) the source of the species that make up the fauna and flora 

 of a zone or area and the centers of dispersal from which they 

 have spread. 



Dr. Spencer Trotter* has pointed out that a zoogeographic (or 

 phytogeograplhic) map shows only a transitory condition, and 

 that the boundaries of zones and the ranges of species are always 

 changing, the rate of change corresponding with the rate of 

 physical or climatic change which the earth's surface may be 

 undergoing. 



Now, in studying plant distribution it seems to me we are con- 

 stantly coming upon facts that bear upon conditions previous to 

 those now existing; the local nature of the plant making such 

 cases much more numerous than those that we find among verte- 

 brate animals. And most of the apparent irregularities of plant 

 distribution — isolated colonies, etc. — may safely be regarded as 

 remnants of a former range of the species at a time when 

 different conditions prevailed. 



I might say here, as will be further explained beyond, that I 

 do not consider that the mere presence of similar soil conditions 

 at two remote localities is in itself sufficient to account for a 

 certain resemblance in the floras of the two spots. There have 

 probably been physical or climatic changes which have brought 

 the plants to both these and other regions at some previous time, 

 and they have persisted where soil conditions remained con- 

 genial, and disappeared and been superseded by other plants 

 where conditions were not suited to their needs. 



* Auk, 1909. PP- 231-233- 



