XV 
THE RELATION OF FACTS TO THEORIES 
323 
man will,” the “ man ” in question being apparently 
the all-compelling force of Natural Selection. 
The facts so stated are certainly sufficiently 
remarkable and seem at first sight at least to 
warrant Weismann’s conclusion that they are only 
explicable by the action of Natural Selection, but 
detailed reflection shows many difficulties. The 
prime assertion of the immunity of the Danaidae, etc. 
is denied by many, the persecution of the pro¬ 
tectively coloured butterflies is also, as we have seen, 
doubted by field entomologists. The relation be¬ 
tween the colour of the wings and the position taken 
up by them in repose seems a very striking fact, 
but I have noticed a somewhat similar occurrence in 
the feathers of birds. In the humming-bird Cyno- 
lesbia gorgo the tail is forked and the tail-feathers 
overlap one another ; the tips of the feathers are of a 
gorgeous metallic colour, but this is confined to a 
simple band at the exposed part, the part of the 
feather which is overlapped being a deep black. 
Owing to the forking of the tail, the overlapping is 
such that in each quill more of the vane is covered 
on one side than on the other, the distribution of 
black and metallic colour corresponds exactly to 
this overlapping, so that the metallic colour extends 
farther back on one side of the rachis than on the 
other. Here is a case very like that of the butter¬ 
flies’ wings, and yet it is almost impossible to believe 
that it can have been produced and maintained by 
its utility (see also p. 167). 
We have confined our study of protective colora¬ 
tion and mimicry to the Lepidoptera, because they 
are admitted on all hands to exhibit the phenomena 
