viii INTRODUCTION. VOL. I. 



The grouping of Races into Species, of Species into Genera, and of Genera into Families, 

 though based upon natural characters and relationships, is not governed by any definite rule 

 that can be drawn from nature for determining just what characters shall be sufficient to 

 constitute a Species, a Genus or a Family. These groups are, therefore, necessarily more or 

 less arbitrary and depend upon the judgment of scientific experts, in which natural characters 

 and affinities, as the most important and fundamental factors, do not necessarily exclude 

 considerations of scientific convenience. The practice among the most approved authors has 

 accordingly been various. Some have made the number of genera and families as few as 

 possible. This results in associating under one name species or genera that present marked 

 differences among themselves. The present tendency of expert opinion is to separate more 

 freely into convenient natural groups, according to similarity of structure, habit, form or 

 appearance. While this somewhat increases the number of these divisions, it has the distinct 

 advantage of decreasing the size of the groups, and thus materially facilitates their study. 

 This view has been taken in the present work, following in most instances, but not in all, the 

 arrangement adopted by Engler and Prantl in their great work, " Natiirliche Pflanzen- 

 familien," in which nearly all known genera are described. 



Systematic Arrangement. 



The Nineteenth Century closed with the almost unanimous scientific judgment that the 

 order of nature is an order of evolution and development from the more simple to the more 

 complex. In no department of Natural Science is this progressive development more marked 

 or more demonstrable than in the vegetable life of the globe. Systematic Arrangement 

 should logically follow the natural order; and by this method also, as now generally recog- 

 nized, the best results of study and arrangement are obtained. The sequence of Families 

 formerly adopted has become incongruous with our present knowledge ; and it has for some 

 time past been gradually superseded by truer scientific arrangements in the later works of 

 many authors. 



It now seems probable that continued investigation and consideration will again modify 

 the sequence of various groups. Many suggestions in this regard have already appeared in 

 botanical literature; notably, in our own country, those of Professor Charles E. Bessey. 



The more simple forms are, in general, distinguished from the more complex, (i) by 

 fewer organs or parts; (2) by the less perfect adaptation of the organs to the purposes they 

 subserve; (3) by the relative degree of development of the more important organs; (4) by 

 the lesser degree of differentiation of the plant-body or of its organs; (5) by considerations 

 of antiquity, as indicated by the geological record; (6) by a consideration of the phenomena 

 of embryogeny. Thus, the Pteridophyta, which do not produce seeds and which appeared 

 on the earth in Silurian time, are simpler than the Spermatophyta ; the Gymnospermae in 

 which the ovules are borne on the face of a scale, and which are known from the Devonian 

 period onward, are simpler than the Angiospermae, whose ovules are borne in a closed cavity, 

 and which are unknown before the Jurassic. 



In the Angiospermae the simpler types are those whose floral structure is nearest the 

 structure of the branch or stem from which the flower has been metamorphosed, that is to 

 say, in which the parts of the flower (modified leaves) are more nearly separate or distinct 

 from each other, the leaves of any stem or branch being normally separated, while those are 

 the most complex whose floral parts are most united. These principles are applied to the 

 arrangement of the Subclasses Monocotyledones and Dicotyledones independently, the Mono- 

 cotyledones being usually regarded as the simpler, as shown by the less degree of differentia- 

 tion of their tissues, though their floral structure is not so very different nor their antiquity 

 much greater, so far as present information goes. For these reasons it is considered that 

 Typhaceae are the simplest of the Monocotyledones, and Orchidaceae the most complex; 

 Saururaceae the simplest family of Dicotyledones in our area, and Compositae the most 

 complex 



Inasmuch as evolution has not always been progressive, but some groups, on the con- 

 trary, have clearly been developed by degradation from more highly organized ones, and 

 other groups have been produced by divergence along more than one line from the parent 

 stock, no linear consecutive sequence can, at all points, truly represent the actual lines of 

 descent. 



