INTRODUCTION 1 9 



view in embryology, but are quite near in front 

 of us. 



The Similarity of all young Embryos 



The study of embryology was given a great fillip by 

 the publication, and general acceptance among 

 scientists, of Darwin's theory of evolution. It had 

 already been found that the most general features 

 of an animal's organization, those by which it was 

 classified as a vertebrate, say, were formed early in 

 its development, and only later there arose the more 

 specialized characteristics by which it could be 

 classified as a bird or a mammal, while still later it 

 would develop the particular features of a fowl or 

 a duck. This means that in the very earliest stages 

 in development all embryos only show those charac- 

 ters which are common to all animals. They must 

 therefore look more or less alike. We have only to 

 describe the early stages and on the basis of their 

 common pattern we can make a general scheme 

 into which all embryos fit, and can classify the ways 

 in which they gradually diverge from each other. 



Soon after the publication of Darwin's Origin of 

 Species, Haeckel put forward a general theory about 

 these early similarities. He supposed that each 

 animal, as it develops from the o^gg to the adult, 

 passes through a series of stages, each of which is 

 similar to one of its ancestors in the course of the 

 evolutionary history of the species to which it belongs. 

 The series is more or less in the right order, so that 

 the stage representing the first generalized ancestral 



