CL 4.SSIFICA TION OF FERNS. 23 



sufficiently discussed the arrangement to see that 

 ferns belong to the isosporons division of the group 

 of Vascular Cryptogams. They have, therefore, but 

 one spore, and possess woody bundles in their 

 tissue. And, as we have examined the growth of 

 a fern in Chapter I., we have also discovered that 

 ferns have a visible alternation of generations, as 

 it is called. This means that they are not directly 

 produced from the seed as are flowering plants, 

 but their fertilization takes place by means of free 

 moving bodies (antherozoids) upon minute shield- 

 like structures (prothalli), which were themselves 

 developed directly from the spore, without any 

 fertilization having taken place. 



We now come to look more closely at the ferns 

 themselves. The class Filices is divided into 

 orders, genera, and species. This suggests a pro- 

 found question, which has puzzled wiser heads 

 than will ever trouble themselves to read this 

 book, and one which has been discussed by Dar- 

 win, Huxley, and almost every eminent scientist 

 in the world: What is a species? As it is the 

 unit by which we count in studying any classifica- 

 tion, we need to understand it as clearly as possi- 

 ble. Smith, in his " Historia Filicum," London, 

 1875, says, "The difficulty of defining a species 

 becomes evident on taking a view of the numerous 

 forms which connect one species with another. 

 It will be found beyond human power to ascertain 



