of the Animal Kingdom. g2o 
is the necessary condition, the natural result of the theory of 
evolution. Matters are quite different, however, when we con- 
sider existing nature. We see that existing animals are all 
separated into larger or smaller groups, more or less removed 
from each other. The individual members of these groups 
frequently stand in very close relationship to one another, but 
the groups themselves are generally separated by smaller or 
larger intervals or gaps, and these gaps are now usually not 
occupied by intermediate forms. Every isolated group in the 
system, many families, orders, and classes furnish plenty of 
examples of what has just been said. , 
It would be a great mistake if any one were to conclude at 
once, from the total absence of intermediate forms, that such 
had never existed in nature. As experience shows, it is only 
in very rare cases and under favourable conditions that the 
fossil forms remain quite unaltered, leaving out of considera- 
tion the fact that a great number of them, such as the Ceelen- 
terata, Tunicata, and Protovertebrata, as well as the greater 
part of the Vermes, owing to their softer texture, could not, 
even under the most favourable circumstances, leave any trace 
behind them. 
What great value attaches to the knowledge of paleonto- 
logical facts, and how important it is for the determination of 
the phylogenetic relationships between the different groups, as 
well as for a correct classification, is best proved by the most 
recent position of our system. ‘Thus the character or defini- 
tion of the class of Birds appears something quite different, if 
we take into consideration exclusively the existing forms, or 
if we include the extinct Archwopteryx and the fossil Odont- 
ornithes (toothed birds), lately discovered by Prof. Marsh in 
North America, such as Hesperornis regalis and Ichthyornis 
dispar. Not only in this class, however, but in nearly every 
more or less isolated group, forms occur, the natural position 
of which in the system can only be correctly determined by 
comparison with the known paleontological types. If we 
consider thegreat number of extinct forms the traces of which 
are still unknown to us, we must regard the establishment 
of a complete genealogical and phylogenetic system as still 
impossible, considering the imperfection of our present pale- 
ontological knowledge. Finally, if we think of those delicate 
and soft animal forms whose traces are entirely lost, and which 
therefore will for ever remain unknown to the naturalist, it 
becomes more than probable that we shall really never be in a 
position to establish and completely construct an absolutely 
perfect genealogical tree of the whole animal kingdom. 
For the present, however, we must content ourselves with 
