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 
ensuos 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. Hifect 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 
