1T4 Report of the State Geologist. 



ing in the same series forms quite distinct. Thus, if we should 

 rely exclusively on the form of the limbs in Quadrupeds, we 

 would be led to associate Reptiles like the Ichthyosaurus and 

 Plesiosaurus with the Mammalia like the Whales and Seals, on 

 account of the fact that in them the limbs are transformed 

 into swimming organs. In many cases the inexactitude in the 

 result is less evident than in the example just cited. 



It will then often be found difficult to determine which are 

 the organs whose diverse aspects are a decisive indication of all 

 the stages of the evolution of a group. We should, therefore, 

 turn our attention not to one single organ, but to the whole 

 assemblage of the more important organs. We shall often see a 

 type which, belonging to a determinate group by the sum of its 

 characters, is at the same time separated from it by some one 

 character which we call aberrant. In this case the difficulty 

 can sometimes be easily explained ; in determining the general 

 expression of the evolution we can deduce the particular history 

 of a given organ which presents especial difficulties. 



This method is applicable in Palaeontology only at the cost of 

 great labor. The organs preserved in a fossil state are usually 

 few in number, and it is well understood that it is not always 

 possible to conclude from the external form what the internal 

 structure is. The question of the state of preservation holds an 

 important place, and even under the most favorable circum- 

 stances, great skill is required in order to study, for instance, the 

 brachial organs of the Brachiopods; the masticatory organs 

 and ambulacral zones of the Echini; the ventral surface of 

 the^Crinoids. The discoveries made in this direction, in other 

 words, the application of the method of Comparative Anatomy to 

 fossil forms, has effected a marked progress in Phylogeny. We' 

 shall see a striking example of this in the chapters which treat of 

 vegetable Palaeontology. The knowledge of the organs of repro- 

 duction, the simultaneous study of the stems and roots of the 

 plants of the Carboniferous epoch, have made possible the com- 

 plete anatomical study of these plants; has filled an important 

 gap between the vascular Cryptogams and the Gymnosperms, 

 and has marked out with an almost absolute certainty the general 

 p rogress of the evolution of vegetable forms. 



