RELATIONS OF PALEONTOLOGY TO GEOLOGY. 8$ 



botanist similarly rely essentially upon Morphology in the determina- 

 tion of the relations of animals and plants ; and there is, therefore, 

 no real difference in the methods of study employed, whether the 

 organisms under examination be living or extinct. 



In some respects, however, the zoologist has a great advantage 

 over the palaeontologist. The student of living beings can investi- 

 gate the entire organism, the soft parts as well as the hard : and he 

 can also study the "development" of the organism, and by tracing 

 it through its early stages can discover how it came to assume its 

 adult characters. The student of fossil organisms, on the other 

 hand, is restricted, with the rarest exceptions, to an investigation of 

 the hard parts only. The conclusions of the palaeontologist as to 

 the characters and affinities of fossil animals are necessarily based 

 upon a study of the skeletal structures, from which the characters 

 of the soft parts have to be inferred. Moreover, it commonly 

 happens that even the hard parts of the animal have been im- 

 perfectly preserved, and that the object to be studied is a mere 

 fragment of the skeleton, from which all the soft tissues have been 

 removed. Again, it is only in exceptional cases that we have any 

 means of making ourselves acquainted with the development of 

 fossil animals. Considering the generally fragmentary character of 

 the objects with which the palaeontologist has to deal, and the almost 

 invariable absence in fossils of any traces of the soft parts of the 

 organism, it might be supposed that the study of fossils was attended 

 with insuperable difficulties. The most serious of these difficulties 

 are, however, overcome by means of the law of the " correlation of 

 organs," the establishment of which by the illustrious Cuvier marks 

 an era in palaeontological science. 



Stated in its most general form, the law of the correlation of 

 organs is the law that all the parts of an organism stand in some 

 relation to one another, the form and characters of each part being 

 more or less closely dependent on, and connected with, the form 

 and characters of all the rest. In other words, an organism is not a 

 fortuitous collocation of unrelated parts, but is composed of mutually 

 adapted and related organs, the possession of any given organ, there- 

 fore, implying the possession of other " correlated " organs. Thus, 

 the possession of mammary glands is " correlated " with the posses- 

 sion of two occipital condyles and of a simple mandible ; a stomach 

 adapted for rumination is correlated with the possession of only two 

 functional toes to the foot, and the absence of the central upper 

 incisors ; an inflected angle of the lower jaw is usually correlated with 

 the possession of "marsupial bones" or "marsupial cartilages"; a 

 covering of feathers is correlated (in living forms) with saddle-shaped 

 faces to the bodies of the cervical vertebrae. From the above ex- 

 amples it will be evident that, by means of the law of correlation, it 



