74 ZEBRAS, HORSES, AND HYBRIDS 



way of looking at the world, and his influence was 

 paramount in many schools. The trend which biology 

 has taken since Darwin's time is also partly due to a 

 fervent belief in the recapitulation theory, according 

 to which an animal in developing from the egg passes 

 through phases which resemble certain stages in the 

 past history of the ancestors of the animals. For 

 example, there is no doubt that both birds and 

 mammals are descended from some fish-like animal 

 that lived in the water and breathed by gills borne on 

 slits in the gullet, and every bird and mammal passes 

 through a stage in which these gill-slits are present, 

 though their function is lost, and they soon close up 

 and disappear. In the hope, which has been but 

 partially realized, that a knowledge of the stages 

 through which an animal passes on its path from the 

 ovum to the adult would throw light on the origin of 

 the race, the attention of zoologists has been largely 

 concentrated on details of embryology, and a mass of 

 facts has already been accumulated which threatens to 

 overwhelm the worker. 



The two chief factors which play a part in the origin 

 of species are heredity and variation, and until we 

 know more about the laws which govern these factors, 

 we cannot hope to arrive at any satisfactory criteria 

 by which we can estimate the importance of the data 

 accumulated for us by comparative anatomists and 

 embryologists. Signs are not wanting that this view 

 is beginning to be appreciated. The publication of 

 ' Materials for the Study of Variation ' by Mr. Bateson 

 some years ago shows that there exists a small but 

 active school of workers in this field ; and recent 

 congresses on hybridization give evidence that in 

 America, on the Continent, and in Great Britain, 

 one of the most important sides of heredity is being 

 minutely and extensively explored. Professor Cossar 

 Ewart's experiments, which we shall attempt to sum- 



