196 BIOLOGY AND ITS MAKERS 
mentary organs not adapted to their condition of life. Most 
of the rudimentary organs arc 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 detcrmining 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: (1) 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 betwecn the new 
and the old—the sane thinker, the great observer. 
