230 BIOLOGY AND ITS MAKERS 
pearance, but disappear long before birth. These indica- 
tions, and similar ones, must have some meaning. 
Now whatever qualities an animal exhibits after birth 
are attributed to heredity. May it not be that all the inter- 
mediate stages are also inheritances, and, therefore, represent 
phases in ancestral history? If they be, indeed, clues to 
ancestral conditions, may we not, by patching together our 
observations, be able to interpret the record, just as the his- 
tory of ancient peoples has been made out from fragments 
in the shape of coins, vases, implements, hieroglyphics, in- 
scriptions, etc. ? 
The Recapitulation Theory.—The results of reflection in 
this direction led to the foundation of the recapitulation 
theory, according to which animals are supposed, in their 
individual development, to recapitulate to a considerable 
degree phases of their ancestral history. This is one of the 
widest generalizations of embryology. It was suggested in 
the writings of Von Baer and Louis Agassiz, but received its 
first clear and complete expression in 1863, in the writings of 
Fritz Miiller. 
Although the course of events in development is a record, 
it is, at best, only a fragmentary and imperfect one. Many 
stages have been dropped out, others are unduly prolonged 
or abbreviated, or appear out of chronological order, and, 
besides this, some of the structures have arisen from adapta- 
tion of a particular organism to its conditions of develop- 
ment, and are, therefore, not ancestral at all, but, as it were, 
recent additions to the text. The interpretation becomes a 
difficult task, which requires much balance of judgment and 
profound analysis. 
The recapitulation theory was a dominant note in all 
Balfour’s speculations, and in that of his contemporary and 
fellow-student Marshall. It has received its most sweeping 
application in the works of Ernst Haeckel. 
