60 OUR NATIVE FERNS AND THEIR ALLIES. 



CHAPTER IX. 

 DISTRIBUTION IN TIME AND SPACE. 



139. Geographic Distribution. Ferns are found in all 

 parts of the world. The number of described species is not 

 certainly known, and the uncertainty is largely increased for 

 the reason that our best systematists do not agree as to what 

 constitutes a species. Baker, whose authority is generally 

 recognized in England and America, places the estimate at 

 about 3000 species. Added to these are 565 fern allies as recog- 

 nized by the same author. 



From what has been said respecting the climatic conditions 

 of fern growth we would naturally expect to find them most 

 abundant in countries where warmth and moisture predomi- 

 nate. These conditions seem most completely met on tropical 

 islands or in tropical continental areas with insular climates. 

 The little island of Mauritius, having an area of 676 square 

 miles, or less than one third the area of Delaware, has 235 na- 

 tive species, while Java, little larger than New York, has 460. 

 Brazil furnishes 387, and the Isthmus of Panama 117. Com- 

 paring these with colder climates, we find 67 in all Europe, and 

 only 26 grow within the borders of the arctic zone. 



" Our Native Ferns," as described later in this volume, in- 

 cluding those species that are classed in the order FILICES, 

 number 156 species. Adding to these the 11 species of the 

 order OPHIOGLOSSACE^E, which have frequently been enumer- 

 ated with the ferns, we have a total of 167 species. The remain- 

 ing fern allies number 56, making a grand total of 223. 



1 4-O. Divisions of our Flora. It has been found conven- 

 ient to divide the surface of the earth into faunas and floras, 

 limited by the natural distribution of the various species of 

 animals and plants. These limits are by no means sharply 

 defined, for wherever the limit is made some species will pass 



