58 THE EVOLUTION OF MAN. 



elementary parts, and in the segments of its composite 

 organs, in a word, in the degree of histological and mor- 

 phological differentiation. The type, on the other hand, 

 is the order of the arrangement of the organic elements and 

 of the organs. The type is quite distinct from the degree of 

 perfection; the same type may exist in several degrees 

 of perfection ; and, conversely, the same grade of perfection 

 may be reached in several types. This explains the phe- 

 nomenon that the most perfect animals of any type, for 

 example, the highest Arthropods and Molluscs, are much 

 more perfectly organized, or more highly differentiated, 

 than the most imperfect animals of other types, for ex- 

 ample, than the lowest Vertebrates and Star-animals. 



Baer's Law has been of the greatest importance in 

 advancing our knowledge of animal organization ; though 

 it was not until a later period that Darwin enabled us to 

 perceive and value its real significance. Here we may 

 at once remark that it can only be really understood by 

 means of the Theory of Descent, by the recognition of the 

 very important part played by Heredity and Adaptation 

 in the construction of organic form. As I have shown in 

 my Generclle Morphologie (vol. ii. p. 10), the type of 

 evolution is the mechanical result of Heredity; the degree 

 of perfection is the mechanical result of Adaptation. 

 Heredity and Adaptation are the mechanical factors in the 

 production of organic forms, which were first brought to 

 bear on Ontogeny by Darwin's Theory of Selection, and 

 which have enabled us for the first time to understand 

 Baer's Law. 



Baer's labours marked the beginning of a new epoch, 

 and aroused an extraordinary interest in embryological 



