22 
feation ; and this teaching has been widely spread by the 
writings of Cuvier, Owen, Hugh Miller, and Murchison. It " 
has been common to point to the notochordal vertebral column 
of palaeozoic fishes, to the ichthyic characters of the liassic 
reptiles, to the numerous identical body-segments of the trilo- 
bites, and ask w^hether all this does not imply an embryonic 
and undifferentiated grade of life as characteristic of early 
geological periods. But these views will be put to a fair test 
if we compare nearly allied animals and plants of different 
ages, noting their resemblances and differences, and estimating 
their modifications with reference to progressive develop- 
ment. Do we find the Silurian calymene to be more embry- 
onic than its Devonian congener ? Are any more recent 
trilobites so highly differentiated as agnostm ? Are the 
carboniferous representatives of the order in any respect 
more perfect than those of Cambrian age ? Or take the 
brachiopoda — what mesozoic or tertiary form is so comj)lex 
as the palaeozoic sjnrifera? In what particulars does the 
tcaldheimia of our Australian seas exhibit a higher organisa- 
tion than the ivaldheimia of the oolite? Among fishes — to 
what more differentiated forms have the palaeozoic ganoids 
given origin? What evidence do forms of long duration, 
such as lepidotiis and hcryx, give of progressive improvement? 
Examined in this manner the generalisation would soon take 
a different shape. Professor Huxley has briefly reviewed the 
evidence in his anniversary address to the Greological Society 
(1862). Among invertebrata he finds no support for the 
doctrine. Of the vertebrata two groups seem to furnish 
satisfactory examples of progressive modification of the verte- 
bral column, in correspondence with geological age, viz., the 
pycnodonts among fishes and the labyrinthodonts among 
amphibia. Professor Huxley rests the latter part of his 
decision upon the fact that, while the carboniferous arche- 
gosaurus had very imperfectly develoj)ed vertebral centra, 
