I9 6 BIOLOGY AND ITS MAKERS 



mentary organs not adapted to their condition of life. Most 

 of the rudimentary organs are transitory, and bear testimony, 

 as hereditary survivals, to the line of ancestry. They are 

 clues by means of which phases in the evolution of animal 

 life may be deciphered. 



Bearing in mind the continually shifting changes through 

 which animals pass in their embryonic development, one 

 begins to see why the adult structures of animals are so diffi- 

 cult to understand. They are not only complex ; they are also 

 greatly modified. The adult condition of any organ or tissue 

 is the last step in a series of gradually acquired modifications, 

 and is, therefore, the farthest departure from that which is 

 ancestral and archetypal. But in the process of formation 

 all the simpler conditions are exhibited. If, therefore, we 

 wish to understand an organ or an animal, we must follow its 

 development, and see it in simpler conditions, before the 

 great modifications have been added. 



The tracing of the stages whereby cells merge into tissues, 

 tissues into organs, and determining how the organs by com- 

 binations build up the body, is embryology. On account of 

 the extended applications of this subject in biology, and the 

 light which it throws on all structural studies, we shall be 

 justified in giving its history at somewhat greater length 

 than that adopted in treating of other topics. 



Five Historical Periods. — The story of the rise of this 

 interesting department of biology can, for convenience, be 

 divided into five periods, each marked by an advance in 

 general knowledge. These are: (i) the period of Harvey 

 and Malpighi; (2) the period of Wolff; (3) the period of 

 Von Baer; (4) the period from Von Baer to Balfour; and 

 (5) the period of Balfour, with an indication of present tend- 

 encies. Among all the leaders Von Baer stands as a monu- 

 mental figure at the parting of the ways between the new 

 and the old — the sane thinker, the great observer. 



