1891.] The Origin of the Galapagos Islands. 313 
considerably, and this change became inherited. Let us con- 
sider what was done in this case. The wild carrot was isolated 
from the others, and brought under different conditions ; it received, 
for instance, more food, and the effect was the change. It 
is exactly in the same way that we have to explain the change 
of forms on the different islands ; an arid, dry island must have 
a different effect on an organism than a fertile and moist island. 
The different condition produces a different effect, and thus a 
different form. If the conditions were absolutely the same, 
the effect would be the same. Let us imagine that we have a 
form A, which has, and so have its ancestors, been for a long 
time in the same conditions. This form A will be represented at 
a certain moment by very numerous individuals of different age, 
between the egg and the senile stage. Now let us change the 
conditions; the change will affect this long series of individuals 
of the same species in a very different way. The new-born will 
react differently from the senile form. But among this long series 
of individuals there will be a certain number of organisms which 
will be most plastic, as I may express it, to the stimulus of the new 
conditions. The senile forms, for instance, probably are not affect- 
ed at all; they die out through senility. Between these and the 
egg-stage, however, certain members must be in the condition 
which I call most plastic. The different individuals may be ex- 
pressed according to their age, A’, A?, A7, A‘, A’, . 
A", the oldest individual, disappears by senility aaa Am takes 
its place ; the whole series is moved one line; the individuals of 
the greatest plasticity, for instance, do not remain the same, but are 
replaced by the next younger group. In this way a constant 
flow takes place, which continues until harmony is rachel 
again between the individuals and the conditions. 
I may explain this a little better by an example. The different 
stages of a plant, from the youngest to the oldest, may be 
expressed in a certain moment by A', A?, A’, At, A5, . . A> 
Now new conditions begin to appear, for instance, W tie grad- 
ual disappearance of water. The result will be that the plant has 
to depend on less food than before; the large forms which have 
_ become so large through ample food will die out through senility ; 
