STRUCTURE AND FUNCTION 21 
of many Podostemaceae, the leaf-like stems of Ruscus and 
the root-like stems of Psilotum. Here we cannot absolutely 
prove that a metamorphosis has occurred during or in 
the phylogeny, but the general evidence favours such an 
assumption. The important point now is that the actual 
function performed by an organ is no proof of its phylo- 
genetic structural nature. The majority of organs now 
absorbing water from the soil are roots, in all probability 
lineally descended from the original roots that were first 
differentiated to perform the water-absorbing function ; but 
other organs are at times found doing this work, with their 
structure correspondingly changed. 
While differentiation and change of function go hand 
in hand with differentiation and change of structure, we do 
not know which is cause and which effect. Most probably, 
perhaps, neither is, but both are phenomena of some more 
general law. They are however the great features of 
evolution and should be studied together 1 . 
We have now to consider how they work. If all organ- 
isms of the same kind were alike in structure and function, 
and offspring like their parents, progress would not be 
possible, but this is not the case, and the variation that 
exists forms the basis for change and evolution. 
Variation. No two organisms or organs are exactly 
alike, even though they be the offspring of the same parent. 
Whatever character be chosen, examination of a large 
number of cases will show that it varies in degree of 
development in different individuals. The study of varia- 
tion has received less attention than it deserves, considering 
that upon it rests the whole theory of evolution ; recently 
however important work has been done. To formulate laws 
of variation in the present state of our knowledge is almost 
impossible, but a few important generalisations have been 
made. 
In any character whose value can be numerically ex- 
pressed, e.g. the height of the individual, it is found that 
1 And see Goebel, Organography of Plants , Engl. ed. ; Asa Gray, 
Structural Botany (formal non-evolutionary morphology) ; Sachs, 
Lectures 07 t Physiology , Engl. ed. (a rebound to the opposite extreme 
of physiological morphology) ; Sachs, History of Botany . 
