1 88 DUBLIN NATTTRAL HISTOET SOCIETY. 



BO much, I went on to say (and I quoted a passage ft'om Hooker's " Spe- 

 cies Filicum" in corroboration) that other observers, who had studied 

 these ferns in many widely separated countries and under various cli- 

 mates, had found the distinctions to be frequently less definitely marked 

 than they are with us ; that the involucre varied from being strongly 

 and equally toothed, to being irregularly toothed or sub-entire; in 

 short, that it put on characters intermediate between those attributed to 

 Tunbridgense on the one part, or to Wilsoni on the other. And I held 

 that if these statements be true, and they are supported by Dr. Joseph 

 Hooker and Mr. Bentham, they were evidence which in my mind 

 weakened the value of the previous evidence, and induced me, therefore, 

 to question the specific distinction of these ferns. They lead me to ex- 

 pect that when more extended and more exact observations are made, 

 even in this country, that intermediate forms of involucre will yet be 

 discovered. And I would particularly recommend those who take an 

 interest in this question to examine the involucres on a great number of 

 fronds of both varieties, selected from plants growing side by side, as 

 they often do, on the same rock, or mixed together, in the same patch — 

 to make careful drawings of all variations in the involucral teeth ; and, 

 above all, when the two occur in one patch, to ascertain that they do 

 not occur on one rhizome. 



I then remarked on the fact stated in the ** Species Filicum," that 

 both these ferns are found almost side by side in a great many distant 

 countries, one frequenting moister and more shady, the other dry and 

 more exposed situations, and that wherever one is found the other may 

 be expected to occur ; and from this I drew a conclusion unfavourable 

 to their specific diversity — such a conclusion as that which would be 

 drawn by most botanists, who regard geographical distribution as deter- 

 mining the distinction of species, if two closely allied fonns present 

 themselves. But if one is found only in the eastern, the other only in 

 the western hemisphere, a botanist will be much more ready to accept 

 them for distinct species than if both are found occuning constantly 

 together, never far apart, not in one place only, but in fifty different and 

 widely separated localities. The fact of their 'hunting in couples,' to a 

 botanical mind, is suggestive of a closer relationship. I should have 

 supposed that a zoologist would have taken a similar view ; but Dr. 

 Kinahan considers this multiplied proximity and constant association 

 afford **an argument in favour of the distinctness of species," for "it 

 is inconceivable," he says, " that the same modifying causes which in- 

 fluence vegetation could exist in all countries where these ferns are 

 found." S'ow I believe that the great " modifjdng causes" which in- 

 fluence vegetation, namely, variations in heat, moisture, sunlight, and 

 exposure to atmospheric changes, do exist, and proportionably too, as 

 respects aspects, in all countries where plants grow. I also believe that 

 some, at least, of the differences indicated between the two Hymcno- 

 phyllums are just such as might be influenced by the ''modifying 

 causes" in question. The softer, more expanded leaves, and broader, 

 more succulent involucres, and the freer growth of H. Tunbridgense, in- 



