VARIATIONS Ol' FUNGI. 55 



Tricholoma from one of the subgenus Clitocybe, because, though 

 one is distinguished by having the gills eraarginate or sinuated 

 behind before their attachment to the stem takes place, and 

 the other has the gills acutely adnate without any emargina- 

 tion, modifications occur on cither side; while in Clitocybe, 

 in an early stage, there may be a decided emargination, in 

 Tricholoma, from the depression of the pileus, the gills may 

 become decurrent. And yet these characters are founded in 

 nature, and are satisfactory enough when the variations to 

 which they are subject are properly appreciated. Still more, 

 changes of outward form may occasionally take place, incon- 

 sistent with the character of the species. Thus we may have 

 umbonate individuals where the pileus ought simply to be 

 obtuse, while a stemless Agaric may exhibit a stem or the 

 contrary. The fact, however, is, that as in phsenogamic 

 botany the sum of characters must be looked to, while it is 

 remembered that no definitions in natural history can be 

 strictly mathematical. Where species are very difficult to 

 distinguish, it is in general because forms are separated which 

 arc too closely allied, an evil which is familiar enough to every 

 practical botanist, though apt to be overlooked or completely 

 ignored by the inexperienced or mere localists. The essential 

 characters are often the least superficial, and hence the young 

 botanist is apt to make mistakes, from confounding mere ana- 

 logies with affinities. Some Agarics of the subgenus Pratella, 

 for instance, would never be separated from others of the 

 subgenus Lepiota, without examining the nature of the spores. 

 These organs, moreover, sometimes differ in closely allied 

 species of such similar external characters, that it would be 

 impossible to distinguish them without having recourse to the 

 microscope. 



If there is difficulty about species, there is often far more 

 about genera. The characters in so natural a group are 



