28G THE COMTARATIVE M0RPH()LO(!Y OF THE CRAYFISH, 



are allied. Given one of those i)rotopIasmic 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 anj^ plant, or of anj^ animal. 



This is the great result of comparative morphology ; 

 and it is careiuUy to be noted that this result is not a 

 speculation, but a generalisation. The truths of anatomy 

 and of embryology are generalised statements of facts 

 of experience ; the question whether an animal is more 

 or less like another m 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 

 animals 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 maj^ 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 b}' no means 

 necessarily true ; and no amount of jDurely 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 



