120 THE DATA OP BIOLOGY. 



though usually not marked between adjacent generations, 

 become great in course of multitudinous generations. On 

 the other hand, we have those developmental modifications, 

 seen in the embryos, through which such modifications of the 

 descended forms are reached. 



Interpretation of the structures of individual organisms 

 and successions of organisms, is aided by two subsidiary 

 divisions of biologic inqidry, named Comparative Anatomy 

 (properly Comparative Morphology) and Comparative Em 

 bryology. These cannot be regarded as in themselves parts 

 of Biology; since the facts embraced under them are not 

 substantive phenomena, but are simply incidental to sub 

 stantive phenomena. All the truths of structural Biology 

 are comprehended under the two foregoing subdivisions; 

 and the comparison of these truths as presented in different 

 classes of organisms, is simply a method of interpreting them. 



Nevertheless, though Comparative Morphology and Com 

 parative Embryology do not disclose additional concrete 

 facts, they lead to the establishment of certain abstract facts. 

 By them it is made manifest that underneath the superficial 

 differences of groups and classes and types of organisms, 

 there are hidden fundamental similarities; and that the 

 courses of development in such groups and classes and types, 

 though in many respects divergent, are in some essential 

 respects, coincident. The wide truths thus disclosed, come 

 under the heads of General Morphology and General Em 

 bryology. 



By contrasting organisms there is also achieved that 

 grouping of the like and separation of the unlike, called 

 Classification. First by observation of external characters; 

 second by observation of internal characters ; and third by 

 observation of the phases of development; it is ascertained 

 what organisms are most similar in all respects; what 

 organisms otherwise unlike are like in important traits; 

 what organisms though apparently unallied have common 

 primordial characters. Whence there results such an ar- 



