24 INTRODUCTION 



of an individual of specific shape and character, to the interactions of 

 certain dispositions of which the adult organism in question is the final result. 

 We must, therefore, take as an axiom the principle involved in the state- 

 ments, that from an acorn nothing but an oak can develop ; that the 

 leaf of an oak has become once and for all different from that of a beech; 

 that the root of the beech-tree develops differently from the shoot ; and 

 that, in general, the specific shape and nature of the organism are inherited 

 properties, or, in other words, that the characters of the parents will be 

 repeated in their offspring. 



Owing to the influence which interaction with the external world 

 exerts, the shape which the plant or any of its parts assumes may vary 

 within certain limits, according to the conditions under which the plant 

 develops. Variation is possible only within certain limits, determined by 

 the extent to which the inherent factors which regulate growth and 

 development may be modified by external agencies. These inherent 

 factors may be termed 'historic or inherited characteristics,' 'specific 

 formative energy,' ' tendency to assume a special shape,' or 'automorphosis' ; 

 but in all cases, provided the continued existence of the organism is 

 possible, the essential features of its inherited characteristics always become 

 manifest whatever the external conditions may be. Individual variations, 

 which may occur under special conditions, are not repeated in the offspring, 

 when these are grown under quite different external environment. This 

 principle is one of general application, although it is especially evident 

 in morphogenetic or formative processes, to which special attention may 

 therefore be paid. Nevertheless, it must be remembered that every morpho- 

 logical variation is an infallible sign that a corresponding modification of 

 the metabolic processes has been initiated. Since every physiological 

 process is dependent upon both internal and external factors, the shape 

 of any organism may be regarded as being due to the interaction of 

 automorphosis and heteromorphosis 1 . 



Every local or individual variation due to the influence of a special 

 habitat is the visible sign of a heteromorphic action. In certain plants, 

 endowed with marked powers of reaction and accommodation, the changes 

 thus induced may be so pronounced that, unless various transition stages 

 were known, the extreme forms would be classified as distinct species. 

 It is sufficient here to recall the aquatic and terrestrial forms exhibited 

 by many plants, as well as the peculiar shapes assumed by many fungi and 

 algae when grown in concentrated nutrient media, or by certain fungi when 

 excited to fermentative activity. Moreover, certain algae may be com- 



To indicate the changes of shape induced by the action of external agencies the term ' hetero- 

 morphosis' or ' xenomorphosis ' may be employed. Sachs used the term ' mechanomorphosis ' in 

 the same general sense, but Herbst (Biol. Centralbl., 1895, Bd. XV, pp. 37-39) has since used the 

 latter term in a more special sense, as referring to the results produced by pressure and tension. 



