38 A CONTRIBUTION TO, ETC. 



systematic arrangement of Ferns according to their species, arises 

 principally from their highly varied forms ; so that in many in- 

 stances, it is almost impossible to determine a true species from a 

 mere variety. In some of our genera, for instance such as Doodia, 

 JBlechnum, and Lomaria, it seems highly probable that, when the 

 vast amount of variation to which species are liable is better un- 

 derstood, the principles laid down by Sir William Hooker will 

 lead to further innovations. Many years ago, the celebrated 

 Robert Brown called especial attention to the position of the 

 sori, or the collections of sporangia, on the veins of Ferns, and 

 this subject was subsequently followed up by J. Smith, A.L.S., 

 in some able papers read before the Linnean Society, in 1840, in 

 which it was shown that the different forms of venation afford, in 

 conjunction with the position of the sori, very important characters 

 for arranging extensive genera. Smith applied his principles in 

 the first instance to the genus Polypodium, and although Hooker 

 regards him as one of those botanists who has addicted himself 

 too much to the multiplication of genera, yet he (Sir William) 

 gratefully acknowledges the benefits which have resulted from 

 these close and accurate investigations ; and in his arrangement 

 of the genus referred to, he adopts the very natural system of 

 grouping the species according to their venation. Thus, for 

 instance, instead of regarding our Polypodium rupestre, and P. 

 tenellum, as species of two different genera (Nipliobolus rupestre, 

 and Arthropteris tenella), he simply places them in sections ac- 

 cording to the character of their veins. The system of making 

 large genera with appropriate subdivisions is, as Dr. F. Mueller 

 has remarked, not merely philosophical, but also highly useful to 

 the memory. Nor, indeed, is it without its importance in reference 

 to the variation of species, for when it is found that forms some- 

 what different in appearance, agree exactly in the ramifications 

 of the veins and the position of the sori, there may be reason to 

 believe that the multiplication of species in such cases is un- 

 necessary. The attentive consideration of the venation also may 

 be useful when we are not so fortunate as to meet with specimens 

 of some Ferns in a state of fructification ; for there are different 

 plants closely resembling each other in habit and outline, which 

 are widely separated by the shape of the veins, some probably 

 having them forked, and others, reticulated. Under any circum- 



