NATURAL SELECTION 229 
selection may modify and adapt the larva of an insect to a score of 
contingencies, wholly different from those which concern the mature 
insect; and these modifications may affect, through correlation, the 
structure of the adult. So, conversely, modifications in the adult may 
affect the structure of the larva; but in all cases natural selection will 
ensure that they shall not be injurious: for if they were so, the species 
would become extinct. 
Natural selection will modify the structure of the young in relation 
to the parent, and of the parent in relation to the young. In social 
animals it will adapt the structure of each individual for the benefit of 
the whole community, if the community profits by the selected 
change. What natural selection cannot do, is to medify the structure 
of one species, without giving it any advantage, for the good of another 
species; and though statements to this effect may be found in works 
of natural history, I cannot find one case which will bear investigation. 
A structure used only once in an animal’s life, if of high importance 
to it, might be modified to any extent by natural selection; for instance 
the great jaws possessed by certain insects, used exclusively for open- 
ing the cocoon—or the hard tip to the beak of unhatched birds, used 
for breaking the egg. It has been asserted, that of the best short- 
beaked tumbler-pigeons a greater number perish in the egg than are 
able to get out of it; so that fanciers assist in the act of hatching. 
Now if nature had to make the beak of a full-grown pigeon very 
short for the bird’s own advantage, the process of modification would 
be very slow, and there would be simultaneously the most rigorous 
selection of all the young birds within the egg, which had the most 
powerful and hardest beaks, for all with weak beaks would inevitably 
perish; or, more delicate and more easily broken shells might be 
selected, the thickness of the shell being known to vary like every 
other structure. 
It may be well here to remark that with all beings there must be 
much fortuitous destruction, which can have little or no influence on 
the course of natural selection. For instance a vast number of eggs 
or seeds are annually devoured, and these could be modified through 
natural selection only if they varied in some manner which protected 
them from their enemies. Yet many of these eggs or seeds would 
perhaps, if not destroyed, have yielded individuals better adapted to 
their conditions of life than any of those which happened to survive. 
So again a vast number of mature animals and plants, whether or 
not they be the best adapted to their conditions, must be annually 
