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38 CHELTENHAM COLLEGE NATURAT HISTORY SOCIETY. 
lie, that you may easily tread upon them by mistake, while the 
pheasant, partridge and wild duck, that have light-coloured eggs, always 
cover them before they go from home. 
Surely these facts are too strong for chance, there are instances of 
protective colouring, of which I should like to be able to speak at 
greater length, but they do not quite come under the head of mimicry 
proper. Mimicry depends on a resemblance between an animal 
and some object among its immediate surroundings, of which it is a 
practical gain to the creature to be a more or less exact copy. For 
instance if a toad so closely resembles a knob of wood that a heron 
fails to see it, when seeking for food, we have a case of true mimicry, 
for the toad saved its life by the imposture. Now of mimicry of this 
kind we have some examples in England. I mentioned to you the 
case of the clear-wing. Several caterpillars so closely resemble the 
food plant, on which they live, as to be almost indistinguishable. 
The Emperor caterpillar, which is found on heather, has alternate 
patches of black and dark green with little red edges, which easily 
pass for heather blossom. And the little geometer caterpillars are 
so called because they rest with their bodies at right angles to a 
branch so as to be almost certainly taken for little dried twigs. 
There is one instance too of a predatory spider mimicking the 
blossom of the guelder rose so exactly that foolish insects settle within 
its deadly clutches in the hope of finding honey. But it is among 
tropical insects that we find the most perfect examples of mimicry. 
Let us notice one or two properties which are common to all the 
mimics. 
(1). They are weak and defenceless, and generally good to eat. 
(2). They move slowly, and depend for safety on fraud rather 
than on speed. 
It is at the same time one of the surest and one of the saddest 
effects of evolution this great law of Nature—that when a creature 
gives up using any of its powers, it soon loses the power altogether. 
I remember when a small boy being very much impressed with a 
story told to me by an old lady. She had a beautiful little clock in 
her room, with a bell alarum beneath, which was set by a string that 
hung out in an irritating way. One day I was caught pulling it, but 
nothing happened. I suppose it was out of order, but my lasting 
impression of her story was this:—Once upon a time she used to get up 
every morning at 7, and then her alarum used to wake her with the 
utmost regularity, but at last she got old and lazy, and then the 
- alarum refused to work when it found it was not listened to. I am 
