220 EVOLUTION IN BIOLOGY VI 
which are closely analogous to those which are 
permanent in some fishes. 
4, That branch of biology which is termed Mor- 
phology is a commentary upon, and expansion of, 
the proposition that widely different animals or 
plants, and widely different parts of animals or 
plants, are constructed upon the same plan. 
From the rough comparison of the skeleton of a 
bird with that of a man by Belon, in the sixteenth 
century (to go no farther back), down to the 
theory of the limbs and the theory of the skull at 
the present day ; or, from the first demonstration of 
the homologies of the parts of a flower by C. F. 
Wolff, to the present elaborate analysis of the 
floral organs, morphology exhibits a continual 
advance towards the demonstration of a funda- 
mental unity among the seeming diversities of 
living structures. And this demonstration has 
been completed by the final establishment of the 
cell theory, which involves the admission of a 
primitive conformity, not only of all the elemen- 
tary structures in animals and plants respectively, 
but of those in the one of these great divisions 
of living thmgs with those in the other. No @ 
priort difficulty can be said to stand in the way of 
evolution, when it can be shown that all animals— 
and all plants proceed by modes of development, 
which are similar in principle, — a fundamental. 
protoplasmic material. 
5. The innumerable cases of structures, which are 
ew are en Tew er ew ere viere® Cowen) Ty PF ‘A 
