Development of Animal Organization. 219 
must be borrowed from a numerical evaluation of natural divi- 
sions, families and groups. 
In these conclusions, however, great care and circumspection 
will always be required, because we shall never be able, by our 
investigation of fossil remains, to acquire a competent know ledge 
of a flora or a fauna of a former period. Ten years ago, a recen- 
sion of all fossil species of extinct animals and plants of the 
different strata was given by Prof. Bronn, of Heidelberg. He 
* then enumerated 708 species of mammals, 148 of birds, 884 of 
reptiles, and 1461 of fishes as fossil. In this recension all the 
different strata are combined and mixed together. When we 
‘compare this general result with an evaluation of the now 
living species of these four classes of Vertebrata, we remark a 
very great difference in the relation of the numbers. The class of 
birds, for instance, in the present period embraces a much greater 
number (perhaps 5 or 6: 1) than that of mammals. In the 
combined faunze of former periods the relation between the spe- 
cies of birds and mammals would be, on the contrary, like 1: 5. 
But still greater would be the difference in the comparative 
numbers of species in the lower classes. Prof. Bronn assumes 
2885 species of fossil Articulata, 13,805 of Mollusca, and 4895 
of Zoophytes (chiefly Echinoderms and Polypes). In the present 
condition of the organic world, the number of known species of 
articulated animals is much greater than that of the Mollusca— 
nay, even than that of all the other classes put together. The 
class of Insects (now so greatly predominant that several orders 
contain myriads of species) is represented in Bronn’s list by 
only 1551 species *. Even when we grant that the relation 
between the numeric value of species belonging to each class 
was different at former periods (and this cannot be denied), we 
must still have recourse to other reasons for the explanation of 
these facts. We must search for another solution of the ques- 
tion why birds amongst the Vertebrata, insects amongst the 
lower animals, have left such a small number of remains in 
comparison with those of fishes and mollusks. Moreover, of the 
fossil remains of insects, nearly all belong to Tertiary periods ; 
Tertiary species of insects form fourteen-fifteenths of the whole 
number. It would be an inconsiderate and highly uncritical 
conclusion, if we were led by this evaluation to the belief that 
the number of insects was so small in former periods, because 
we see so few remains of them in the strata of our rocks. It is 
also clear that the vestiges of Medusze and other soft animals, 
which are so numerous in our seas, may be totally wanting, 
- * Leonhard und Bronn, ‘ Neues Jahrbuch fiir Mineralogie,’ 1849, S. 128 ; 
H. Bronn, ‘ Handbuch der Geschichte der Natur,’ IIIter | Band. Stuttgart, 
1849. 
