VARIATION 
2 3 
It is evident, then, that in studying a plant or an organ, 
we must examine a number of individuals to be sure that 
we are dealing with an average form, not with extremes 
or sports. 
A very important factor in variation problems is what 
is termed correlated variation. By this is meant that when 
an organism varies in one part, the result is frequently a 
variation in other parts as well, often such as have apparently 
no direct relationship to the first. Very probably nutrition 
has something to do with this ; if an extra amount is 
diverted into any channel, as will happen in many cases 
of variation, the balance will have to be redressed else- 
where, and this may be the cause of further variations. 
No rules can be formulated about variation of this kind, 
but there are many cases which seem only explicable by 
assuming correlation. 
As to the causes of variation, we know little. The 
mixing of parental characters in generation, and change 
of external conditions of life, are probably among the most 
active. The extent of the permanent influence of the 
external conditions upon plant-structure has yet to be 
investigated. So far as we can judge, favourable, unfavour- 
able, and indifferent variations appear. Variation thus 
provides a basis for structural and functional change, and 
we have now to consider how the latter may be effected 1 . 
Rate of Increase. Every kind of animal and plant 
produces so many offspring that if nothing interfered with 
their growth and propagation they would soon cover the 
globe. As the average number of representatives of each 
derivation of stamens from petals and so on. Other common mon- 
strosities are fasciation , often found in buttercups, &c. and hereditary in 
the garden cockscomb (Celosia, q.v.), in which the stems and 
branches are laterally united, forming a flat structure, peloria (see 
Scrophulariaceae), and prolijication , or formation of buds in the axils 
of floral leaves. The evidence of monstrosities can only be regarded 
as useful in solving morphological problems when a series of stages can 
be found between the monstrous and the normal forms. 
1 And see Darwin, Origin of Species , Variation of Animals and 
Plants , Descent of Man ; Wallace, Darwinis?n ; Bateson, Variation ; 
Galton, Natural Inheritance ; De Vries, Die Mutations-theorie ; Daven- 
port, Statistical Methods', Vernon, Variation in Animals and Plants ; 
&c. 
