Development of Animal Organization. 219 



most 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 knowledge 

 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, 384 of 

 reptiles, and 1161 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 

 eombined faunae 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 ArticuIaU, 13,805 of MoUusca, 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 MoUusca — 

 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 Medusae and other soft animals, 

 which are so numerous in our seas, may be totally wanting, 



♦ Leonhard und Bronn, 'Neuea Jahrbuch fiir Mineralogie,' 1849, S. 128 ; 

 H. Bronn, * Handbuch der Qeschichte der Natur,' liUer Band. Stuttgart, 

 184y. 



