360 M. L. Agassiz on the Primitive Diversity and 



that this is so far from being the case, that in many instances 

 the reverse is really true. I have already attempted elsewhere 

 to show in outlines what is the real order of succession of the 

 great types of the animal kingdom ; I need not therefore repeat 

 here what may be gathered from the diagram at the head of the 

 Zoological Text Book I have published jointly with Dr. Gould. 

 I shall limit myself to a few more general remarks upon the 

 special difficulties involved in a more thorough investigation of 

 the subject. 



The study of the order of succession and gradation of the 

 organized beings which have inhabited our globe at different 

 periods, presents indeed difficulties of more than one kind. Un- 

 happily, these difficulties have seldom been all considered in their 

 natural connexion by those who have ventured to consider the 

 subject in its whole extent ; thus presenting certain results as 

 general which would require various qualifications to be true. 

 In comparing fossils of one and the same, or of different geolo- 

 gical formations, it is in reality not enough to ascertain their true 

 geological horizon, which we may call the chronological element 

 of the inquiry ; it is equally important that the differences or 

 resemblances arising from the geographical distribution over the 

 wide expanse of the whole surface of the globe, which we may 

 call the topographic element of the question, should be also con- 

 sidered, for it is already known that, within certain limits, the 

 same differences and resemblances which are observed at present 

 between the animals inhabiting different parts of the globe existed 

 already in former geological periods. We must therefore become 

 acquainted with the general biological character of the epoch as 

 well as with the local fauna of each period. The tertiary faunae 

 of New Holland and the Brazils, for instance, resemble more 

 closely the living fauna? of those parts of the world than they 

 resemble one another. Our lists of fossils teem with chronolo- 

 gical errors of the worst kind, arising partly from false identifi- 

 cations of strata, which in reality belong to different periods, 

 but the fossils of which are thus represented as having inhabited 

 our globe simultaneously, when in reality they may have been 

 separated by long periods of time, and existed upon the earth 

 under very different physical conditions. This chronological 

 confusion is further increased by the too extensive limits fre- 

 quently assigned by geologists to the successive groups of rocks 

 forming the crust of our globe. For instance, when the creta- 

 ceous or the oolitic formations are considered respectively as in- 

 divisible natural groups, and the fossils of all their subdivisions 

 are enumerated in one single list as the inhabitants of a long 

 period, an infinitude of anachronisms are presented to the mind, 

 which no special mention of localities can rectify ; and until the 



