THE EVOLUTION AND VARIATION OF FRUIT PLANTS. 21 



of the organism by the external conditions of existence or b}' the 

 influences which surround them. The laws of adaptation, or the 

 facts of variation caused by the influence of outward circum- 

 stances, are as important as the laws of inheritance. All 

 phenomena of adaptation can be traced to states of nutrition of 

 the organism, in the same way as those of inheritance are referable 

 to conditions of reproduction, though both may be traced to 

 chemical or physical causes ; and these new forms of organisms, 

 the transformations which artificial selection produces in the state 

 of cultivation, and that which natural selection furnishes in the 

 state of nature, arise solely by the interaction of these mechanical 

 causes. The functions of inheritance and adaptation have been 

 able so far to produce all the variety of fruit forms, but as to the 

 causes of deviation, naturalists have assumed the interaction of a 

 changing and a preserving formative tendency, corresponding 

 with the processes of adaptation and inheritance. The latter 

 strives to keep the organic form in its species, to form the 

 descendants like the parents, and always to produce identical 

 results; while adaptation, which counteracts inheritance, con- 

 stantly strives to change the organic forms through the influence 

 of the varying agencies of the outer world ; to create modifications 

 of those existing, and to entirely destroy the constancy and 

 and permanency of species. As fruit plants were born with a 

 tendenc}" to vary, so many forms have been slowly evolved during 

 the ages from those which were simpler and older ; and could we 

 trace back their history we should undoubtedly find evidence 

 that in the past they had for a common ancestor species that had 

 not acquired any of their distinctive features ; the characteristics 

 of the wild individual having been caused by its scanty food and 

 constant exposure. The fact that variation is due to the action 

 of changed or unnatural conditions upon certain cells of a preced- 

 ing generation is a reason why wild fruits vary and have individual 

 peculiarities, each plant being under slightly different relations to 

 the external world from all the others ; but as compared to 

 domesticated species their conditions of life are very uniform. 

 Domestic varieties var}' more than their wild relations ; and fruit 

 plants long propagated and cultivated by seed are highly variable, 

 owing to changed conditions of life. The tendencj' to vary is in 

 itself hereditary ; one variation is the cause of anotlier variation ; 

 a change in one cell of an organism will disturb the harmonious 

 adjustment of all related cells. 



