HETEROCHROSIS IN ESSEX BIRDS AND THEIR EGGS. 179
Reconsideration of Parts 1 and 2.
We have in the first instance discussed some of the more
practical aspects of heterochrosis as it affects birds and then, so
that this phenomenon might be intelligently considered, we have
found it necessary to outline the nature and causes of colours
in the plumage of birds. Before we proceed to deal with the
question as it particularly affects Essex it may be well to recapi-
tulate briefly.
Of the four phases of colour variation albinism is much the
commonest. Its cause or causes are still of a theoretical nature,
and we do not understand fundamentally the differences between
complete and partial albinos. The following practical points
appear to have been established :
1. That of the total albinistic cases recorded not more than
10% are complete.
2. That dark plumages are more subject than those that are
light.
3. That red and green plumages are more resistent.
4. That the pattern of the feather is not obliterated in
albinistic birds.
5. That feathers structurally modified for colour production
lose their special structure when affected by albinism.
6. That albinos may arise from normal parents and continue
through successive generations.
7. That complete albinism is congenital.
8. That partial albinism may be congenital.
9. That congenital partial albinism may revert to normality.
10. That partial albinism may be developed.
11. That developed partial albinism may revert to normality.
12. That partial albinism may persist or increase for years
until the death of the bird.
13. That some birds, including those of normal plumage,
have a predisposition to beget white young.
14. That albinism may be inherited through several genera-
tions, but not in a fixed manner as though controlled by some
law of nature.
15. That albinism may be inherited from one parent, either
male or female.
On the other hand certain points remain to be determined;
among the more noticeable are the following :