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THE POPULAR SCIENCE MONTHLY. 



show this relationship as conclusively as the vertebrate relationship 

 of birds and mammals is shown by the presence of a backbone in 

 each of them. It will be seen that the facts in this imaginary case 

 belong to the same category with the facts of homology, but that they 



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furnish a much more complete index of relationship, since they cover 

 the whole life of the animal, instead of its adult form alone. 



As a matter of fact we do find in nature something like this hypo- 

 thetical case, and it is universally recognized that an acquaintance 

 with all the stages in the growth of an animal is the greatest aid to 

 the discovery of its true affinities ; as is well shown by the case of 

 entoconcha, and by the barnacles which were classed among the mol- 

 lusca until a knowledge of their development showed that they are 

 Crustacea. When descriptive embryology was in its infancy, it so fre- 

 quently happened that a knowledge of younger stages threw a flood 

 of light upon the affinities of doubtful forms, that naturalists felt a 

 growing hope that here was the true key to the natural system of 

 classification, and that all they needed for reading the riddle was a 

 thorough knowledge of the whole course of development of each form 

 of life. If the embryology of each animal were a fixed quantity, 

 this purely descriptive knowledge would undoubtedly furnish such a 

 basis for phylogenetic generalizations ; but the great advances which 

 have been made in this field within the last twenty years show conclu- 

 sively that this is not the case, but that the early stages in the life of 

 an animal may undergo modifications which are quite independent of 

 any which may meanwhile take place in the adult, so that, while the 



