644 HISTORY OF SCIENCE, 



doctrine of motor and sensory nerves established by ist, the dis- 

 coveries of CHARLES BELL, published in his " Anatomy of the Brain " 

 (1811) and "Arrangement of the Nerves" (1821); 2nd, those of 

 MAJENDIE ; 3rd, those of JOHANNES MULLER ; and the theory of re- 

 flex action due to the independent labours of the last-named physio- 

 logists, and of MARSHALL HALL (1832) ; and the more recent labours 

 of FLOURENS, BROWN-SEQUARD, FERRIER, and many others. Not a 

 little general interest attaches also to the results of modern researches 

 into the physiology of sensation by Young, PURKINJE, BONDERS, 

 Miiller, WEBER, Helmholtz, and others. We must be content to 

 represent all these last by the brief reference made at page 445 to some 

 of Young's work in this direction. 



In the systematic Botany of the present century the chief name is 

 AUGUSTIN PYRAMUS DE CANDOLLE (1778 1841), who was Professor 

 of Botany at Montpellier in 1807, and at Geneva in 1816, and pub- 

 lished his "Elementary Principles of Botany" in 1804. His system 

 was a modification of that of Jussieu (page 388). It made its way but 

 slowly. In England it was first brought into notice by ROBERT BROWN 

 (1773 I ^s8), who employed it in his "Flora of New Holland," pub- 

 lished in 1 8 10. This system, in which plants are grouped according 

 to their general affinities of form, structure, and vital functions, is with 

 slight differences that adopted at the present day by ENDLICHER, 

 LINDLEY, HOOKER, BENTHAM, etc. In classification, the fundamental 

 question is of course the relationships of varieties, species, and genera. 

 The older views regarded variation as taking place within certain limits 

 only ; and it was supposed that varieties necessarily tended to revert 

 to the original stock. At the present day, most naturalists regard 

 variation as continuous, and hold that, under appropriate external con- 

 ditions, the variations of the descendants from a common stock may 

 accumulate to constitute the difference of species. The five following 

 propositions embody Mr. George Bentham's definitions of varieties, 

 species, and genera in the vegetable kingdom : 



1. Although the whole of the numerous offspring of plants resemble their parent in 

 all main points, there are slight individual differences. 



2. The great majority of the few which survive and propagate in their turn, are under 

 ordinary circumstances those which most resemble their parent : thus the species con- 

 tinues without material variations. 



3. There are occasions, however, when individuals with slightly diverging characters 

 survive, and reproduce races in which these divergences are continued with ever increased 

 intensity : hence the origin of varieties. 



4. In the course of numberless generations, circumstances may increase the divergency 

 so far that the varieties will no longer propagate with each other : thus the varieties 

 become species. 



5. Species have in turn become the parents of groups of species ; that is, of genera, 

 orders^ etc. , of a grade higher according to the remoteness of the common parent, and 

 more or less marked according to the extinction or preservation of the primary or inter- 

 mediate forms. 



Vegetable Physiology had, at the close of last century, made but 



