ON VARIETIES, EACES, SUB-SPECIES, AND SPECIES. 83 



people began to raise fruit-trees from seed ; the two mo^t cele- 

 brated persons of this class were Van Mens in Belgium, and 

 Sageret in France, and of these the former made the experiment 

 on the largest scale, whilst the latter published the most on the 

 subject, having laid before tlie woi'ld all his researches made from 

 the year 1794 down to our own day, in his celebrated treatise 

 Pomologie Physiologique, and the memoirs which preceded and 

 followed that remarkable work. 



We have mentioned tlie sowing of several plants as good ex- 

 amples of the distinctions above applied to species, regarded in 

 relation to the subordination of the groups of individuals com- 

 posing them respectively ; we have still to examine how plants 

 may be modified by having their seeds continually sown accord- 

 ing to the usual methods practised by gardeners for the purpose 

 of obtaining varieties. It may be as well, however, to explain 

 first of all the exact meaning intended to be conveyed when we 

 speak of the possibility of modifying certain plants in order to 

 obtain varieties by sowing their seeds. 



The sowing of seeds is not the essential cause of the modifica- 

 tions which may then be manifested, but it is only an occasional 

 cause, as we shall show hereafter when we review the general 

 causes of these modifications. 



There are two periods in the life of plants which, in studying 

 their modifications, require special notice. 



The Jirst period commences with the formation of the seed, 

 and finishes the moment it is ripe and can detach itself, or be de- 

 tached from tlie organ bearing it. 



The second period comprises the germination of the seed, and 

 the complete development of the plant proceeding from it. 



After having spoken of general effects, of the causes which 

 act in the two periods of the life of a given individual, we shall 

 treat of the mutual modifications of two organic forms represent- 

 ing a species, when two individuals of different sexes belonging 

 to these species are capable of producing a hybrid. 



We have then to consider : — 



The modifications which take place in individuals of a single 

 species ; 



The modifications of two organic forms as found in a hybrid 

 produced from two individuals of different species. 



1. Modifications of individuals belonging to a single species. 



a. Modifications which a plant can undergo in the Jirst 



period of its existence. 



Seeds taken from an ovary at the same time may, under the 



same circumstances, so far as the sowing and external agents are 



concerned, produce individuals .sufficiently diflTerent to enable us 



G 2 



