History of tlie Theory of Heredity. 25 



proportions of his hull a little to fit it for sailing instead 

 of rowing, that he puts in masts and spars, and makes 

 everything ready for a ship's rigging; that he then 

 changes the shape of his hull once more; tears out part of 

 his cabin, puts in bulkheads, coal bunkers, and an engine 

 and boiler ; shortens his masts, alters his rigging, and 

 finally converts his unfinished ship into a finished 

 steamer. 



This is not by any means a forced illustration, but 

 a very fair outline of the development of an animal. 

 In nearly every case we find that the development of 

 the embryo as a whole, or else the development of cer- 

 tain organs, takes place in this roundabout, indirect way, 

 and repeats, usually in an imperfect manner, the struc- 

 ture of a related but lower animal. 



As an example, we may refer to the history of the 

 blood-vessels of a mammal. The breathing organs of 

 the lower vertebrates are gills on the sides of the neck, 

 and the venous blood is driven from the heart through a 

 series of branchial arteries to the gills, where it is aerated 

 and conveyed into a series of branchial veins which carry 

 it, not back to the heart, but to the various organs of the 

 body. In a mammal there are no traces of gills at any stage 

 of development; the adult animal breathes by lungs, and 

 the blood which has been aerated in the lungs goes back 

 to the heart before it is distributed throughout the body. 

 Now the early stages in the development of the blood- 

 vessels of a mammal would, if carried out to completion, 

 lead to the formation of the system found in fishes. 



The mammalian embryo has no gills, but it does have 

 branchial arteries and veins, and its blood at first follows 

 the same course that it follows in a fish. It is plain 

 that the fish-like circulation is not an outline or sketch 

 of that of a mammal; that it is not a necessary stage in 



