LL, Agassiz on Animals in Geological Times. 323 
change the aspect and relations of the faune of the paleozoic 
rocks. How different, again, would the order of succession of 
Mollusks appear, were we to adhere to Cuvier’s view of separa- 
ting the Brachiopods, as a class, from the other Acephala, to 
which they are now more correctly referred The vexed question 
of the period of appearance of Dicotyledonous plants in the ge- 
ological series would have been settled long ago, had it been 
placed-upon its real foundation. It is not in reality to be argued 
upon paleontological evidence chiefly, for it resolves itself in the 
main into a botanical question, and the definite answer must de- 
pend upon the position finally assigned by botanists to the fami- 
lies of Coniferee and Cycadex. If these natural orders of plants 
are really allied to the Dicotyledonz, then this type begins with 
the paleozoic rocks in the Devonian system, and there is no gra- 
dation in the order of succession of plants during geological times. 
But if the view of Brongniart is more correct, if the Coniferee and 
Cycadex have to be separated from the Dicotyledcne as Gymnos- 
perme, and if moreover these latter should prove, as I believe 
they are, inferior even to the Monocotyledouez, then we may at 
once recognize in the vegetable kingdom a similar gradation of 
types as among animals. ese examples may suffice to show 
what is required for a proper investigation of the order of succes- 
sion of organized beings in the course of time, and how little con- 
ence the investigations in this field deserve, which have not 
been made with due reference to all the points mentioned above. 
It is indeed only in the classes, the structure and embryology of 
Which is equally well understood, we are able to discover the 
laws regulating the succession of animals and plants in geological 
ations, and our knowledge is at present still too imperfect to 
catry the investigation into all families of the animal kingdom. 
And yet enough is known to leave no doubt as to the final result ; 
We may confidently await the time when the glory of the won- 
derful order of creation shall be fully revealed to us, and this may 
stimulate us to renewed efforts, since the success depends entirely 
Upon our own exertions. 
The geographical distribution of animals began only to be 
Studied long after systematic zoology had made considerable pro- 
gress, but even to this day the limits of the faunz are nowhere 
circumseribed with any kind of precision, the principles upon 
which they might be determined are in many respects questiona- 
ble, anda large number of animals are daily described without 
any reference to their natural distribution upon the earth; though 
Much has already been done since Buffon to place this branch of 
our knowledge upon a better foundation, and especially to ascer- 
tain the laws regulating the geographical distribution of certain 
sses and families considered isolately. ‘The point which re- 
quites now particular attention, is the combination of these differ- 
