Change of External Conditions. 1 4 ^ 



structure, will be driven from the scene by more ad- 

 vantageously organised competitors.-' 



If, for example, a species of plant spreads into a district 

 where it finds itself exposed to new kinds of attack, or 

 if, in a district where the species arose, and where until 

 now the external conditions have been suitable to it, 

 these external conditions alter, it may happen that 

 this species will become rarer and rarer, and gradually 

 quite die out. By these alterations of external con- 

 ditions we are not, however, simply to understand 

 changes of climate ; for no less important are the 

 changes which may occur within a given area in the 

 animal world. Quite independently of the changes which 

 occur in their area of distribution, animals as well as 

 plants are subject to variation ; and individual varieties 

 may arise with new characters, that under the special 



1 That structures which, owing to external conditions, have 

 become useless or even prejudicial to a plant, thereon undergo 

 gradual degradation or abortion, is a notion which must be most 

 decidedly rejected. The so-called process of "adaptation" is never 

 a direct one, never comes simply in response to a want. In other 

 words, external conditions can never occasion an inheritable change 

 of form, whether advantageous or the contrary, can neither deter- 

 mine the development of an organ nor its abortion. Structural 

 pecuUarities, advantageous and disadvantageous, present themselves 

 in individual varieties, quite independently of any direct influence 

 of external conditions. When the peculiarity is an advantageous 

 one, its possessor, being thereby well suited for the struggle vdth 

 its local competitors, can maintain its position, can multiply, ex- 

 tend itself, and form the starting-point of a species. When, on the 

 other hand, the peculiarity is one that, under the special local cir- 

 cumstances, confers no advantage, or is positively injurious, its 

 possessor succumbs ia the struggle for existence. 



