168 NATURAL HISTORY NOTE. 
turbances, and so on until the whole organism is modified 
and equilibrium restored. Or it may be that the disturbance 
has been too great and the organism dies. 
Equilibration may be direct or indirect. 
It is direct when the new force calls forth the counteracting 
force, indirect when it is balanced by a change which it has 
no share in producing. 
Direct equilibration is usually spoken of as adaptation. It 
occurs when the force operates on individuals continuously or 
frequently, not inflicting vital injuries. In this manner light 
and heat act on the stems and leaves of plants, and the skin 
of animals ; the effects of use and disuse of organs also must 
be classed under this head. 
Indirect equilibration is a result of the process known as 
Natural Selection. Many beneficial modifications cannot be 
directly produced by the environment. The propensity of 
cattle to browse on a plant would not tend to clothe it with 
thorns or stinging hairs; nor would the tastes of pollen- 
carrying bee or butterfly help to develop honey in a flower. 
The individuals of a species become unlike by the trans¬ 
mitted effect of the joint variations of the parents ; a com¬ 
plex train of new variations being set up in the offspring. If 
any change occurs in the environment some of these individuals 
will be better able than others to accommodate themselves to 
it, and will have a better chance of surviving and producing 
offspring to which their peculiarities will be transmitted. 
Thus a type will gradually be formed in harmony with the 
new conditions. Even where there is no alteration in external 
forces, a more delicate adjustment to existing conditions may 
still be possible, and those individuals which show this 
adjustment are likely to survive their less fortunate fellows. 
A sqiecies may thus be regarded as a moving equilibrium, 
yielding in the direction of least resistance and regaining its 
balance by a compensating reaction. 
Leafing of Oak and Ash. —Luring the first and second weeks of 
May I had several opportunities of observing the progress of foliation 
in these trees over a great part of North Warwickshire. Although 
individual exceptions were numerous, yet it was obvious that by far 
the great majority of the oaks were about a week or ten days in 
advance of the ashes. If the proverb were true this would presage a 
warm and dry summer. The exceptions are due to the constitutional 
differences of individuals. We are familiar with such variations in the 
different kinds of garden peas and strawberries, &c. ; and I am 
acquainted with a horse-chestnut, in Edgbaston, which now for many 
(perhaps eight) years I have observed to be in full leaf more than a 
week before all others in its vicinity.—W. B. Grove. 
