The Making of Species 
of explanation; but, on the assumption that a 
slight rearrangement of the biological atoms in 
the molecule may produce very diverse results, as 
we see in the case of chemical molecules, and 
of seasonally dimorphic butterflies, there is 
no particular ground for surprise at such a 
phenomenon. 
In this connection we may cite the significant 
fact, so well known to canary breeders, that two 
crested birds when mated tend to produce a bald- 
headed one. 
If the colour of any part of an organism be 
due to the internal arrangement of the constituent 
parts of the biological molecule from which it is 
derived, we should expect any rearrangement of 
the component parts to produce quite a different 
colour. In other words, we should expect occa- 
sionally to see colour-mutations. These are pre- 
cisely what we do see. Similarly, if the scheme 
of colouring of an organism be due to a certain 
grouping of biological molecules, we should ex- 
pect the same scheme of colouring to occur in 
organisms which are not nearly related. This, 
too, we observe in nature. 
Many of the phenomena of mimicry, and all 
the cases which we have cited as pseudo-mimicry, 
seem to us to be referable to this. 
Take, for example, the magpie colouration in 
birds—that is to say, a scheme of colouring in 
which the body is white, and head, wings, and 
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