Relation between External Influences and Development 229 



environment, and as the result of this it develops along a certain 

 direction ; it may, for example, become a flower. The particular 

 direction of development is determined before the rudiment is 

 differentiated and is exerted so strongly that further development 

 ensues without interruption, even though the external conditions 

 vary considerably and exert a positively inimical influence : this 

 produces the impression that development proceeds entirely inde- 

 pendently of the outer world. The widespread belief that such 

 independence exists is very premature and at all events unproven. 



The state of the young rudiment is the outcome of previous 

 influences of the external world communicated through other organs. 

 Experiments show that in certain cases, if the efficiency of roots and 

 leaves as organs concerned with nutrition is interfered with, the 

 production of flowers is affected, and their characters, which are 

 normally very constant, undergo far-reaching modifications. To find 

 the right moment at which to make the necessary alteration in the 

 environment is indeed difficult and in many cases not yet possible. 

 This is especially the case with fertilised eggs, which in a higher 

 degree than buds have acquired, through parental influences, an 

 apparently fixed internal organisation, and this seems to have pre- 

 determined their development. It is, however, highly probable 

 that it will be possible, by influencing the parents, to alter the 

 internal organisation and to switch off development on to other 

 lines. 



Having made these general observations I will now cite a few of 

 the many facts at our disposal, in order to illustrate the methods and 

 aim of the experimental methods of research. As a matter of con- 

 venience I will deal separately with modification of development and 

 with modification of single organs. 



i. Effect of environment upon the course of development. 



Every plant, whether an alga or a flowering plant passes, under 

 natural conditions, through a series of developmental stages charac- 

 teristic of each species, and these consist in a regular sequence of 

 definite forms. It is impossible to form an opinion from mere obser- 

 vation and description as to what inner changes are essential for the 

 production of the several forms. We must endeavour to influence 

 the inner factors by known external conditions in such a way that the 

 individual stages in development are separately controlled and the 

 order of their sequence determined at will by experimental treat- 

 ment. Such control over the course of development may be gained 

 with special certainty in the case of the lower organisms. 



With these it is practicable to control the principal conditions of 

 cultivation and to vary them in various ways. By this means it has 



