THE PTERIDOPHYTE DIVISION 549 



closely akin to those which do, have more or less hysterophytic 

 nurse-plants. 



In endeavoring to trace the evolution of fernworts we con- 

 tinually encounter the question as to whether a given type 

 or organ of relatively simple form is best regarded as primi- 

 tive or degenerate. The evidence available is often conflict- 

 ing and has led different botanists to very diverse conclu- 

 sions regarding the kinship and evolution of the different 

 groups. Another stumbling block has been the difficulty of 

 distinguishing between the resemblances that arise from 

 similar adaptation to the same environment though in 

 different lines of descent, and those resemblances which are 

 due to inheritance even under cUfferent environments. It 

 is now generally admitted, however, that the fernworts of 

 the coal period attained much higher development than any 

 which have survived, and that several important features, 

 such as the development of macrospores, have evolved in- 

 dependently in each of the three classes. We have thus 

 good reason to suppose that the progressive evolution of 

 the Pteridophyta was mainly accomplished in geological 

 ages long past, that this progress took place along the three 

 main lines represented by our modern ferns, scouring-rushes, 

 and lycopods, and that these fernworts of to-day are the 

 more or less degenerate descendants of giant plants like those 

 preserved in coal. 



When we take the adder-tongue fern as possibly represent- 

 ing the sort of plant which first evolved from a liverwort 

 like Anthoceros, we must accordingly make allowance for 

 considerable modification of detail due to special adaptations 

 in the course of ages and to more or less degeneration. We 

 cannot say much more than that our supposed ancestor of 

 the ferns presumably had rows of large sporangia along the 

 edges of the blade-like expansion of a growing axis which 

 put forth, below, cylindrical projections for absorbing water. 

 From such an ancestor, ferns, scouring-rushes, and club- 

 mosses may perhaps be supposed to have evolved, one or 

 another according as the stem-parts or leaf-parts reached 

 greater or less, or about equal development, and the spore- 

 sacs were multiplied or reduced in number and diminished 



