30 THE HISTORY OF CREATION, 
has been of the very greatest importance for the phylogeny 
of the whole class of birds. Atl birds previously known 
presented a very uniformly organized group, and showed no 
striking transitional forms to other vertebrate classes, not 
even to the nearly related reptiles. But that fossil bird 
from the Jura possessed not an ordinary bird’s tail, but a 
lizard’s tail, and thus confirmed what had been conjectured 
upon other grounds, namely, the derivation of birds from 
lizards. This single fossil has thus essentially extended not 
only our knowledge of the age of the class of birds, but also 
of their blood relationship to reptiles. In like manner our 
knowledge of other animal groups has been often essentially 
modified by the accidental discovery of a single fossil. The 
palzontological records must necessarily be exceedingly im- 
perfect, because we know of so very few examples, or only 
mere fragments of very many important fossils. 
Another and very sensible gap in these records is caused 
by the circumstance that the intermediate forms which con- 
nect the different species have, as a rule, not been preserved, 
and for the simple reason that (according to the principle of 
divergence of character) they were less favoured in the 
struggle for life than the most divergent varieties, which 
had developed out of one and the same original form. The 
intermediate links have, on the whole, always died out 
rapidly, and have but rarely been preserved as fossils. On 
the other hand, the most divergent forms were able to main- 
tain themselves in life for a longer period as independent 
species, to propagate more numerously, and consequently to 
be more readily petrified. But this does not exclude the 
fact that in some cases the connecting intermediate forms 
of the species have been preserved so perfectly petrified, that 
