30 THE HISTORY OF CEEATION. 



has been of the very greatest importance for the phylogeny 

 of the whole class of birds. All 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 

 palgeontological 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 diflferent 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 



