IIo THE MEANING OF EVOLUTION 
taken many generations, and is very slow, but is none 
the less sure in the end. In most cases the animal is 
probably entirely unconscious of this point in its 
favor, and usually it does nothing to assist the decep- 
tion. The result is none the less effective because the 
animals themselves are unconscious of the process. 
The cabbage worm is green in color like the cabbage. 
This does not mean that it got green by eating cab- 
bage or by longing for greennesses. Through long 
years the enemies of the cabbage worm have been 
picking it off the plants on which it fed. This does 
not imply that cabbages as we know them are very 
old, but cabbage worms doubtless ate the leaves of the 
sea-kale long before man had cultivated it into cab- 
bage. During all these years the enemies of the cater- 
pillars, generally in the shape of birds, have been as- 
siduously gathering them up. 
When we see how much the various members of 
the same human family may differ in complexion, how 
much the various pigs in the same litter may differ 
in size and in coloration, it is easy to understand that 
among these caterpillars which have eaten the cabbage 
there must have been considerable color variations. I 
do not imagine for a moment that the birds had any 
preference for any particular color in their cabbage 
worms. They took every caterpillar they saw, but 
they naturally first saw those that were least like the 
