2 INTRODUCTION 



of Flowering Plants ; and it is improbable that any living forms remain 

 to be discovered differing in any material point of structure from those 

 already known. Here, therefore, we are able to discuss systems of 

 classification which claim something like finality ; and the difficulty of 

 the compiler of a handbook is the enormous amount and the minute 

 detail of the material to his hand, from which he has to cull those por- 

 tions which seem suitable for his object. In order not to extend this 

 portion of the work beyond due limits, it has been necessary frequently 

 to practise rigid compression — beyond, probably, what many of our 

 readers specially interested in these groups would have desired. The 

 same remarks apply, to a large extent, to the Muscineae. But in the 

 ThalhDphytes, and especially in the lower Algae and Chlorophyllous 

 Protophyta, the case is very different. From the extremely minute size 

 of many of these, and the much smaller extent to which they have been 

 studied, new forms are constantly being discovered, and important ad- 

 ditions are yearly being niade to our knowledge of their life-history and 

 of their structure. It is highly probable that among these groups, as 

 well as m some of the orders of Fungi, forms will yet be discovered 

 which cannot be assigned to any type at present known, gaps in the life- 

 history of many species will yet be filled up, and organisms hitherto 

 placed in widely separated families will ultimately be found to be phases 

 in one cycle of development. We have therefore, in this branch of our 

 subject, brought before our readers every fact of importance known to 

 us which is vouched for by observers in whom we have confidence ; 

 and the classification here submitted is a purely tentative one. In the 

 Algse, the Fungi, and the Protophytes, we do not attempt an exhaustive 

 enumeration of orders or families which shall include every known 

 organism, but describe in detail only those types which are of greater 

 importance, and of which our knowledge is more complete. 



Something must be said on the classification adopted. In the Vascular 

 Cryptogams and in the Muscinese this proceeds on generally recognised 

 Hnes, in which there is not much room for difference of opinion. But 

 a very different treatment seemed necessary of the Thallophytes, and of 

 the relationship to one another of the Algae and Fungi, and of the 

 different orders within each of these groups. Here the systems pro- 

 posed are almost as numerous as the original investigators, and it has 

 been necessary to choose that which appeared to the authors to bring 

 together those organisms which are most nearly related to one another. 

 Whether these two familiar terms represent a natural bifurcation in the 

 classification of the lower organisms, is a question which has been 

 very variously answered by different observers and theorisers. About 

 fifteen years ago a system of classification of the Thallophytes was pro- 



