THE THEORY OF LAMARCK 39 
in the position of the eye could be of no advantage 
so long as this organ remained upon the under surface. 
The very young fish, whilst still symmetrical, are 
known sometimes to fall upon one side, and when in 
this position to twist the lower eye forcibly upwards. 
Darwin himself therefore supposed that the origin of 
the adult structure is to be attributed to the ened 
effect of efforts of this kind. 
The interest of the last case lies in the fact that it 
relates to a structure, the origin of which does not 
appear explicable on the theory of natural selection ; 
its bearing will therefore be better understood when 
we come to discuss that theory in the next chapter. 
The inherited effects of voluntary striving can clearly 
have no application to the case of plants. Lamarck 
therefore supposed that evolution in the vegetable 
kingdom had taken place entirely through the action of 
external agencies upon plants. The soil, for example; 
in which a plant grows has a direct influence upon its 
form. Altitude, moisture, heat, and light are other 
important factors, and the effect of their influence upon 
the plant was supposed by Lamarck to be inherited. 
The shape of irregular flowers was regarded as having 
been directly caused by the strains and pressures 
occasioned by bees and other insects whilst making 
their visits in search of honey or pollen. 
Lamarck’s theory turns entirely upon the question 
whether acquired characters are inherited, and if so,; 
to what extent, sime, if such inheritance is shown to 
be extremely slight, the cause, though a true one, 
may be insufficient to explain the effects attributed 
