INTRODUCTION. XXlll 



does, Development what it luas, and how from 

 what it was it came to be what it is. 



The study of development is, however, not 

 merely desirable in itself; it is also absolutely 

 requisite in the determination of homologies. 

 Organs are usually said to be homologous when 

 they correspond with one another in structure, 

 and are connected with similar parts of their re- 

 spective fabrics. But it would appear that these 

 tests are sometimes fallacious, and, in the case of 

 adult organisms, their application often becomes 

 impossible. It is therefore necessary to compare 

 the organs of different animals in their simplest 

 condition, and to trace the several stages through 

 which they pass, before it can be said that they 

 agree with one another. And the same rule may 

 be extended to the entire animals of which such 

 organs form part. 



A knowledge of development alone is capable 

 of furnishing a solution to the inquiry. How far 

 may unity of organisation be predicated of all 

 animals ? The answer is, in their primitive con- 

 dition only, since the study of Morphology clearly 

 shows the existence of distinct plans of structure 

 among adult animals. All these start, as it were, 

 from the same point, namely, the condition of the 

 germ, and from this they rapidly diverge, each 

 successive step in their development tending to 

 separate them farther from one another. To the 



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