516 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



nisliing how many species are referrible, without doubt, to one 

 common type. Thus, ScMzwa flabellum and S. dichotoma, 

 with many varied intermedials, notwithstanding tlieir im- 

 mense difference of form, beyond all doubt, belong to one 

 and the same species. But there are other cases in which, 

 thoagh the identity is not so evident, the limits are absolutely 

 incapable of definition. Asplenium monanthemum* tricho- 

 TYianes, ohlongatwin, and viride, all run into each other, and 

 the same may be said of numerous other groups of species. 

 The only difference between A. ohtusatum and A. marinum, 

 a plant of extremely wide distribution, is the confluence of the 

 upper pinnge into one broad terminal pinna ; in other respects 

 they are absolutely the same. Take, again, such species as 

 Asplenium bulhiferwni and laxum, supposed once to con- 

 stitute two distinct species, till intermediate forms appeared in 

 Tasmania, and compare a good series of both with A. fiaccidum 

 and it will be impossible to say of certain individuals to which 

 species they belong. Such examples might with ease be mul- 

 tiplied. That circumstances will alter the appearance of species 

 is clear from the fact that Loinaria Patersonii, which is cul- 

 tivated at Kew, produced for years simple fronds, but at length 

 became divided ; and I have seen in the same garden the 

 minute pinnules of a Gleichenia expanded to three times their 

 normal length and breadth, and the margin at the same time 

 unfolded, when placed in a hot damp atmosphere (Fig. 110, b). 

 57<S.f " There are few or no natural orders of plants so emi- 

 nently sensitive to changes in the amount of vapour in the 

 atmosphere as the Ferus ; and we consequently find them to 

 be, to a remarkable degree, natural hygrometers, and their 

 luxuriance a certain proof of the dampness of a climate. 

 Moisture, both of the soil and air, appears, indeed, absolutely 

 to regulate their development, both in luxuriance and abun- 



* This species has sometimes three or more sori. 



t I am indebted to Dr. J. D. Hooker for this valuable account of the 

 distribution of ferns, which would have been much extended did the 

 limits of this volume permit. This, however, is far from being the only 

 instance in which I am obliged to his vast fund of knowledge and 

 experience. 



