Roijal Microscopical Society. 205 



When the vegetable kingdom is reached, the natural orders of 

 plants, or the divisions and subdivisions used by botanists as exhibit- 

 ing natural affinities, are those which I believe should certainly 

 regulate the arrangement of a histological collection. It may here 

 be observed, and I should wish to lay particular stress on the cir- 

 cumstance, that whilst the main arrangement is a botanical one 

 founded on natural affinities, the subsidiary divisions moreover 

 ought to be of a physiological character. In this way, not only 

 the differentiation of elementary structures is grouped progressively, 

 but the separation and complexity of organic function is specially 

 defined. 



An essential difficulty in rigidly carrying out such a natural 

 and structural classification of vegetable organisms or botanico- 

 physiological arrangements, is the fact, that the lower forms can be 

 viewed in their completeness in a single slide; while the higher 

 forms, both on account of their size and segregation of organs, 

 require many subseries to illustrate their anatomy. 



This difficulty of arrangement, which equally applies to the 

 animal kingdom, makes it essential that the lower forms, almost up 

 to the ferns, should primarily follow their natural generic affinities, 

 but their subsidiary divisions nevertheless be subject to physiological 

 principles. On the contrary, the higher forms. Monocotyledons and 

 Dicotyledons, should primarily be divided j)hysiologically, and second- 

 arily botanically. Further exemplified ; the Koot, Stem, Flower, 

 Fruit, or organs of nutrition — skeleton, reproduction, &c., could not 

 be arranged separately in every individual order or genera of Dico- 

 tyledons without producing a conflicting confusion in the classification. 



Among the Diatomace^e, for example, many genera may occupy 

 only one slide. In such a case the most numerous forms or fewer lead- 

 ing characteristic ones, point where ought to be the true place of the 

 slide. Again, it may so happen that a set of slides are mounted for 

 the express purpose of demonstrating points connected with a par- 

 ticular locality. It would be a loss rather than a gain, therefore, to 

 break up and distribute such a series, unless means (of which I shall 

 hereafter mention) be taken to preserve their recognition as belong- 

 ing to the original set in question. Diatomaceous earths grouped 

 geographically or according to their geological position, stratigra- 

 phically, will teach their distribution and range in time. 



Parasitical forms, such as Fungi, are usefully and natiu'ally 

 grouped according to the situations they are found in. But in all 

 cases series of the genera and species should take precedence. 



The changes coincident with development should form a series 

 and subsection under the difierent groups. 



In the division of Monocotyledons, Dicotyledons, &c., as above 

 stated, the several organs, or apparatus, circulatory, respiratory, 

 and such like, form the chief feature of arrangement. But fossil 



