VARIOUS CAUSES OF GROWTH. 
777 
These suggestions must suffice here. The extremely difficult question which 
has been raised may be illustrated by far-fetched and elaborate explanations, but 
cannot be satisfactorily answered. 
The external or physical causes of growth are the only ones that can be 
submitted to direct experimental investigation ; the internal hereditary causes must 
be considered simply as something that exists and that is in the main unalterable ; 
for if it were possible to change some of the mechanical and chemical properties 
of a tissue by means of external influences, this could not affect the true kernel 
of the hereditary characteristics; and again conversely, changes in these hereditary 
pecuharities, or varia/ions, are never brought about by direct external influences, 
but only by unknown internal changes. Since therefore the specific peculiarities 
in the organisation of a plant are something in its nature that is entirely unknown, 
any investigation of the processes of growth must rest satisfied with showing the 
mode in which they are always associated with constant internal conditions, and 
what visible changes are produced in the processes of growth by physical influences. 
We cannot therefore be astonished if in the action of known external causes — 
light, gravitation, &c. — on plants, eff'ects are produced which appear altogether 
strange to one accustomed to examine purely physical processes ; but this aston- 
ishment disappears when it is borne in mind that the specific organisation of a 
plant itself represents a complexity of causes which we cannot analyse, and there- 
fore are unable to estimate. It is in the constant recognition of this unknown 
factor— which causes physiological effects to turn out so entirely different from 
purely physical ones — that the difference between physiology and physics consists. 
The most striking mode however in which the aggregate of conditions of growth 
manifests itself in the inherited organisation, is when the same external causes pro- 
duce entirely opposite effects on plants belonging to different species and even on 
different parts of the same plant. 
To understand correctly the phenomena of vegetation, it is also necessary to 
distinguish between the direct and indirect action of external conditions on growth. 
For since growth is always dependent primarily on the presence of assimilated 
food-materials, light, temperature, or other external conditions may indirectly in- 
fluence growth by affecting the formation and transport of the food-materials. 
But it is also possible and even probable that the mechanical process of intus- 
susception itself on which growth is directly dependent may be modified by those 
and other causes the influence of which on growth is therefore in that case a direct 
one. The growth of one part may also be indirectly promoted or retarded by the 
growth or the removal of another part. 
The unknown factor which exists in the inherited properties of organisms is by 
no means without analogy in inorganic nature. Chemists and physicists have also to 
assume peculiar properties of elementary substances. The aggregate of properties by 
which a particle of iron is absolutely distinguished from a particle of oxygen is as 
unknown and much more invariable than the aggregate of physiological causes which 
distinguish the inherited properties of an Oak from those of a Pine. 
So far as the definition given above of historical properties concerns the inherited 
specific peculiarities of plants, the expression is not a metaphor from the point of 
view of the Theory of Descent, but must be taken in its literal signification. The 
specific properties which determine qualitatively the growth of each organ have 
