INTRODUCTION 33 



illustrating a tendency towards the production of new forms the 

 list is open to no such objection. 



II. Climatic Factors 



52. In considering the effect of climate on the distribution of our 

 flora we have to remember the salient fact, that, while it has not 

 been so much of an ancient factor in deciding the general composi- 

 tion of the area as edaphic influences have been, it is very much 

 of a controlling agency at the present day. Even in such a 

 limited area as this there appear to be well marked climatic 

 barriers, through which certain species are scarcely ever known to 

 go. 



53. To dispose at once of rainfall and the winds, which, in a 

 temperate climate such as ours, are almost negligible, it is only 

 necessary to record that the amount and distribution of the rain- 

 fall is such that, in any one part of our area, as against any other 

 part, the differences are so slight, so much above minimum 

 requirements, and so far below a maximum of the rain-forest 

 conditions of the tropics, that it can be ignored; and that we 

 have nothing in any way suggesting an aeolian influence affecting 

 the distribution of our plants, with the possible exception of the 

 purely local sand-drifting along the coast dunes. The highly sug- 

 gestive results obtained by some observers, on the distribution 

 of our native flora as affected by the varying degree of evaporation 

 of available water, are not yet sufficiently comprehensive to be 

 used in the present work. 



54. The chief climatic factor then is temperature, and in at- 

 tempting to arrive at some conclusion as to its effect on the dis- 

 tribution of the plants growing within the area many interesting 

 problems have arisen. The most obvious method of taking the 

 annual mean temperature as a basis of calculation comes to nothing 

 as the differences in this are too slight to account for the very 

 different vegetation in the Catskills where the mean temperature 

 is 45 , and at Cape May, N. J., where the mean temperature is 

 only 53 . The comparative similarity in the temperatures of the 

 two places mentioned does not begin to express the great dis- 

 similarity in the vegetation, nor does this similarity of mean tem- 

 perature imply anything like a sufficiently operative climatic 

 barrier, to maintain the status quo of the vegetation, so to speak. 



4 



