Degeneration of disused parts 39 
change. Nor do we know how far a determinant must be strengthened 
by the passive flow of the nutritive stream if it is to be beyond the 
danger of unfavourable variations, or how far it must be weakened 
passively before it loses the power of recovering itself by its own 
strength. It is no more possible to bring forward actual proofs in 
this case than it was in regard to the selection-value of the initial 
stages of an adaptation. But if we consider that all heritable varia- 
tions must have their roots in the germ-plasm, and further, that. when 
personal selection does not intervene, that is to say, in the case of 
parts which have become useless, a degeneration of the part, and 
therefore also of its determinant must inevitably take place ; then we 
must conclude that processes such as I have assumed are running 
their course within the germ-plasm, and we can do this with as much 
certainty as we were able to infer, from the phenomena of adaptation, 
the selection-value of their initial stages. The fact of the degeneration 
of disused parts seems to me to afford irrefutable proof that the 
fluctuations within the germ-plasm are the real root of all hereditary 
variation, and the preliminary condition for the occurrence of the 
Darwin-Wallace factor of selection. Germinal selection supplies the 
stones out of which personal selection builds her temples and palaces: 
adaptations. The importance for the theory of the process of degenera- 
tion of disused parts cannot be over-estimated, especially when it 
occurs in sterile animal forms, where we are free from the doubt as to 
the alleged Lamarckian factor which is apt to confuse our ideas 
in regard to other cases. 
If we regard the variation of the many determinants concerned in 
the transformation of the female into the sterile worker as having 
come about through the gradual transformation of the ids into 
worker-ids, we shall see that the germ-plasm of the sexual ants must 
contain three kinds of ids, male, female, and worker ids, or if the 
workers have diverged into soldiers and nest-builders, then four 
kinds. We understand that the worker-ids arose because their 
determinants struck out a useful path of variation, whether upward 
or downward, and that they continued in this path until the highest 
attainable degree of utility of the parts determined was reached. 
But in addition to the organs of positive or negative selection-value, 
there were some which were indifferent as far as the success and 
especially the functional capacity of the workers was concerned : 
wings, ovarian tubes, receptaculum seminis, a number of the facets of 
the eye, perhaps even the whole eye. As to the ovarian tubes it 
is possible that their degeneration was an advantage for the workers, 
in saving energy, and if so selection would favour the degeneration ; 
but how could the presence of eyes diminish the usefulness of the 
workers to the colony? or the minute receptaculum seminis, or even 
