ON VARIETIES, lUCES, SUB-SPECIES, AND SPECIES. 109 



by the division of an individual always resemble that individual, 

 and although we never resort to this method of propagation if 

 we wish to obtain varieties, yet it must not be forgotten that the 

 plant separated from its parent may become modified by external 

 causes. If this were not so, we could never explain the changes 

 which take place in vines propagated by cuttings, or the changes 

 a graft undergoes when removed to a country differing from 

 that in which its parent grew, and which graft, taken back to 

 its native country, will also reassume its primitive characters. 

 We do not wish, however, to insist upon this as an absolute 

 principle, and independently of the time in which the modifying 

 external causes are capable of acting, because we cannot but 

 admit that changes which have taken place in an organized body 

 may remain, at least for a certain time, under circumstances 

 other than those which caused them. 



If Knight's opinion on the duration of plants propagated by 

 division be not, in reality, incompatible with the principle of 

 the immutability of species, even when it is admitted that plants 

 live longer than the individual from which they came, yet M. 

 Puvis has developed this view to such a degree that he has com- 

 promised the principle of the immutability of species under 

 actually existing circumstances. 



It was partly to discuss this question, and to develope our own 

 notions on the influence of external agents on living bodies, that 

 we availed ourselves of the opportunity offered us of examining 

 Count Odart's Ampelography, and appended, in the above way, 

 the researches of M. Sageret and the work of M. Puvis. 



By far the greater number of the facts relating to plants on 

 which we have relied have been ascertained by horticulture. 

 This branch of agricultural science ought to engage our best 

 attention, if we may judge of what it can do by what it already 

 has done for as. 



The extent of horticulture is indefinite ; it includes fruit-trees, 

 vegetables, and every edible plant that can live in our gardens 

 or hothouses. Compared with agriculture, which in a given 

 country is applicable to but very few sorts of plants, horticulture 

 is concerned with a vastly greater number of species and varieties. 



The object of horticulture being not only to prolong the life 

 and multiply specimens of all the plants with which it is con- 

 cerned, but also to obtain as many modifications of them as 

 possible, recourse is had to more numerous and varied methods 

 and schemes than are necessary for the purposes of agriculture. 

 The temperature of the air must be made to Aary, and its 

 moisture be confined to the neighbourhood of a few plants: an 

 endless variety of manure and soil is requisite for the different 

 purposes of the gardener. Horticulture is constantly making 



