THEORIES OF EVOLUTION. 
An Historical Outline. 
AGNES ROBERTSON, D.Sc. 
(Continued from page 215.) 
IV.—EMBRYOLOGY. 
Embryology is a source from which some help is derived in 
the study of evolution. Every animal and plant begins life as 
a unicellular organism, but whereas some continue unicellular to 
the end of their lives, others go through developmental processes 
of varying degrees of complication before they reach the mature 
form. Evolutionists believe that the whole organic world has 
been evolved from some very simple form of life, perhaps not 
differing greatly from certain unicellular organisms of the 
present day, so that in phylogeny (the ancestral history of the 
species), and ontogeny (the developmental history of the 
individual), the starting point and the goal are the same, 
which suggests the tempting view that the path between is the 
same also. An idea of this kind was put forward in 1810 by 
Oken, and later more definitely enunciated by Von Baer. The 
‘recapitulation theory,’ as it is called, states that in the 
developmental stages passed through by an embryo we can 
recognise the different stages passed through in previous ages 
in the evolution of the species, or in the words of the epigram- 
matist, ‘Every animal in the course of its development climbs 
up its own genealogical tree.’ But this conclusion can only be 
accepted with great reservations, since at the best embryology 
only gives us a much abbreviated and altered picture of the 
phylogenetic history, for the embryo attains its end by many 
short cuts, and ancestral traits are often obscured by adaptations 
which fit the immature organism for the special conditions of 
its life. 
V.—HEREDITY AND HYBRIDISATION. 
It has always been a matter of common observation that 
children resemble their parents, but it is only of late years that 
scientific methods have been brought to bear upon the subject 
of heredity. In early days the mechanism by which characters 
1907 July 1. 
Q 
