64 THE THEORY OF EVOLUTION 



ture, which was described in the last lecture. The 

 egg, or the active portion of it, divides in a definite 

 and regular manner into a very large number of cells, 

 which arrange themselves in definite layers, an outer 

 and an inner, and within these layers cell-aggregates 

 form incipient organs, which, step by step, take on 

 the adult condition. Not only is the plan and type of 

 development essentially similar throughout the whole 

 phylum of the vertebrates, but, in accordance with 

 the recapitulation theory, many structural features 

 which are permanent in lower forms appear hi the 

 embryos of higher and more advanced types. In 

 the latter, however, these features are transitory and, 

 in the course of development, they either disappear, 

 or are so modified as to be very different, sometimes 

 unrecognizable, in the adults. 



At a certain stage of the ontogeny the embryo of a 

 mammal has gill-pouches like a fish, the skeletal 

 supports of the gill-pouches, the arteries and veins 

 which supply them with blood, the structure of the 

 heart, in short, the entire plan of the circulatory 

 system is fish-like. At a later stage most of the gill- 

 pouches have been obliterated, but one is retained 

 and converted into the Eustachian canal, which 

 connects the throat with the middle ear, inside of the 

 ear-drum. Similarly, the embryological evidence 

 shows that the lungs of air-breathers have been 

 derived from the swim-bladder of fishes, a conclusion 

 which had already been reached by comparative 

 anatomy, for in a remarkable group, known as the 



