986 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
are allied. Given one of those protoplasmic bodies, of 
which we are unable to say certainly whether it is animal 
or plant, and endow it with such inherent capacities of 
self-modification as are manifested daily under our eyes 
by developing ova, and we have a sufficient reason for 
the existence of any plant, or of any animal. 
This is the great result of comparative morphology; 
and it is carefully to be noted that this result is not a 
speculation, but a generalisation. The truths of anatomy 
and of embryology are generalised statements of facte 
of experience ; the question whether an animal is more 
or less like another in its structure and in its develop- 
ment, or not, is capable of being tested by observation ; 
the doctrine of the unity of organisation of plants and 
anjmals is simply a mode of stating the conclusions 
drawn from experience. But, if it is a just mode of 
stating these conclusions, then it is undoubtedly con- 
ceivable that all plants and all animals may have been 
evolved from a common physical basis of life, by pro- 
cesses similar to those which we every day see at work 
in the evolution of individual animals and plants from 
that foundation. 
That which is conceivable, however, is by no means 
necessarily true; and no amount of purely morpho- 
logical evidence can suffice to prove that the forms 
of life have come into existence in one way rather 
than another. 
There is a common plan among churches, no less than 
