MIGRATION OF BIRDS. 149 
and seem to leave altogether, and other birds migrate in 
families; but what, in fact, we want is a theory that will 
explain the facts, and if that of tradition is disproved, as it 
now seems to be, I can only see that we are thrown back 
on a Theory of Heredity, and I rather think that in the end 
the influence of heredity is to explain all the mysteries. 
But the science of the subject is yet in its infancy, and the 
work is too great for one man to work out; areas of 
distribution will have to be defined differently from what 
they are at present. If you consult the bird-cases in the 
Museum you will see little maps coloured red showing the 
distribution areas of the families of birds. These will have 
to be improved on the principle of our contour maps, and 
that in two directions. First, on one coloured map I should 
have the colours varied in proportion to the number of 
species: that might be a clue to the original centre of 
distribution. It would at least have this valuable commen- 
dation, that it would tell the student where the species were 
most numerous. On the second coloured map I should 
have it told at once to the eye what biological research has 
to say regarding primary forms and their developments; we 
could not go further back than that in discovering the 
original centre of distribution. Take, for example, the Thrush 
family, which is cosmopolitan. Of these there is no doubt that 
the ground thrushes are the primary forms, and these are 
found in the tropical regions of Africa and Asia, etc. 
It is well that we go on amassing our facts, but it is not for 
scientific men to tell us to defer making theories till we have 
got the facts; the imagination has its work to do all along 
the investigations, and after all facts are supposed to be 
gathered, it is the imagination that will have to form the 
theory. Facts without imagination are ultimately of little 
scientific use, and even the person that is crying yet for 
facts more and facts only, shows thereby that he is possessed 
of a theory if he only knew what it was. 
At this stage let me give some quotations, Brehm 
says :—“ Hunger and love dominated the migratory move- © 
ments of birds.” These forces alone would not suffice, for 
birds migrate often long before winter sets in, and the passion 
of love does not possess many birds till their second and third 
