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Again, living animals are unceasingly undergoing a change of 

 structure, or, in other words, they have a development; and this 

 development may be anatomically considered at any stage, and 

 for any fraction of time. This constitutes developmental anatomy; 

 and in a similar way we have the province of developmental his- 

 tology. The two combine in a special branch, or that of embry- 

 ology, or the early stages of the development of the individual. 

 Fossil anatomy takes account of the structure of the remains of 

 extinct animals, and incidentally of their history in time. This 

 is the science of palaeontology. The facts brought to light by the 

 study of embryology and palaeontology largely assist in the cor- 

 roboration of a natural taxonomy. 



The path followed in a natural classification of animals is that 

 one which first takes into consideration the very simplest forms 

 known, and passes to those of gradually increasing complexity. 

 In this course, structural resemblances constitute, when taken as 

 a whole, the main guide and basis, checked, as ever must be the 

 case, by what embryology and palaeontology have to offer. 



In writing upon taxonomy, Huxley has truly said that it " is 

 conceivable that all the forms of life should have presented about 

 the same differentiation of structure, and should have differed 

 from one another by superficial characters, each form passing by 

 insensible gradations into those most like it. In this case tax- 

 onomy, or the classification of morphological facts, would have 

 had to confine itself to the formation of a serial arrangement 

 representing the serial gradation of these forms in nature. 



" It is conceivable, again, that living beings should have dif- 

 fered as widely in structure as they actually do, but that the in- 

 terval between any two forms should have been filled up by an 

 unbroken series of gradations; in which case, again, classification 

 could only effect the formation of series the strict definition of 

 groups would be as impossible as in the former case. As a 

 matter of fact, living beings differ enormously, not only in differ- 

 entiation of structure, but in the modes in which that differentia- 

 tion is brought about; and the intervals between extreme forms 

 are not filled up in the existing world by complete series of gra- 

 dations. Hence it arises that living beings are, to a great extent, 

 susceptible of classification into groups, the members of each 

 group resembling one another, and differing from all the rest, by 

 certain definite peculiarities. 



" No two living beings are exactly alike, but it is a matter of 



