282 DEFINITE ACTION OF CONDITIONS OF LIFE. Chap. XXIH. 



throughout Europe and North America, and on which a 

 single bud suddenly produced the red magnum bonurn. We 

 should also bear in mind that distinct varieties, and even 

 distinct species, — as in the case of peaches, nectarines, and 

 apricots, — of certain roses and camellias, — although separated 

 by a vast number of generations from any progenitor in 

 common, and although cultivated under diversified conditions, 

 have yielded by bud-variatiou closely analogous varieties. 

 When we reflect on these facts we become deeply impressed 

 with the conviction that in such cases the nature of the 

 variation depends but little on the conditions to which the 

 plant has been exposed, and not in any especial manner on 

 its individual character, but much more on the inherited 

 nature or constitution of the whole group of allied beings to 

 which the plant in question belongs. We are thus driven to 

 conclude that in most cases the conditions of life play a 

 subordinate part in causing any particular modification ; like 

 that which a spark plays, when a mass of combustibles bursts 

 into flame — the nature of the flame depending on the com- 

 bustible matter, and not on the spark. 63 



No doubt each slight variation must have its efficient cause ; 

 but it is as hopeless an attempt to discover the cause of each, 

 as to say why a chill or a poison affects one man differently 

 from another. Even with modifications resulting from the 

 definite action of the conditions of life, when all or nearly all 

 the individuals, which have been similarly exposed, are simi- 

 larly affected, we can rarely see the precise relation between 

 cause and effect. In the next chapter it will be shown that 

 the increased use or disuse of various organs produces an 

 inherited effect. It will further be seen that certain varia- 

 tions are bound together by correlation as well as by other 

 laws. Beyond this we cannot at present explain either the 

 causes or nature of the variability of organic beings. 



83 Professor Weismann argues 'Saison-Dimorphismusder Schmetter- 

 strongly in favour of this view in his linge,' 18~5, pp. 40-43. 



