28 INTRODUCTION' TO CRYPTOGAMIC BOTANY. 



claims to admittance into the series of cryptogams, and would 

 imdoubtedly vindicate these claims, were it sure that the 

 bodies contamed in their cavity were spores, the membranous 

 coats which Ime their cavity are still as flexible as they could 

 have been in life, exhibiting every marking and peculiarity of 

 structure with the greatest precision, and in some cases, pre- 

 senting a double lamina capable of separation at the point of 

 confluence, while each possesses its own cellular arrangement. 

 It is amongst fossil plants, if anywhere, that we must look 

 for indications of the highest perfection attainable by Crypto- 

 gams. Meanwhile, an accurate acquaintance with the habits 

 of those species which still exist upon the earth, and of 

 the temperature and climate in which they flourish, may 

 possibly afford some key to the questions which so often occur 

 to the geologist, respecting the climate which must have 

 existed on the earth at the time of some given geological 

 formation, to allow of the luxuriant development of these gigan- 

 tic forms. Nor are these the only kind of Cryptogams which 

 abound in certain strata ; there are numerous others which are 

 immediately and certainly comparable with existent species. 

 Many forms occur which have no modern analogues, or are 

 in such an altered state, as to allow of little more than con- 

 jecture as to their affinities ; and even the ferns themselves, 

 whose form is often so beautifully preserved, so very rarely pro- 

 duce fruit, as to lose much of the interest they would otherwise 

 possess ; but the siliceous cases of Diatomacece are preserved in 

 such extreme perfection, as to be as satisfactorily comparable 

 with existent species as the remains of moUusca, and, as in those, 

 a host of species may as safely be pronounced identical. 



17. Another excellent inducement to the study of Crypto- 

 gams is the fact that so many of the diseases, both of plants 

 and animals, arise from their presence. The species which 

 affect animals are probably few in number, and for the 

 most part of common kinds, possessing great powers of ubiquity, 

 and therefore able to establish themselves on what, from the 

 very nature of things, cannot be their natural habitat. Though 

 great attention has been paid to the study of such Cryptoo-ams 

 as infest man, and other animals, they have seldom been 



