250 DARWINISM AND THE PROBLEMS OF LIFE 



impossible to gather the ancestral history from it. 

 But the effect will increase if many of the organs of the 

 ancestors are no longer necessary, and are not found in 

 the adult animal. It is true that these will still have 

 to appear in a rudimentary fashion in the embryonic 

 course, but those animals will be steadily selected in 

 which they take a shorter time and are less in size, as 

 they then leave more room to the other organs. In the 

 end they will be forced out of the embryonic develop- 

 ment altogether. This must, of course, take place 

 gradually, and if we find the gill-clefts of a fish still 

 appearing, and then disappearing, in the human embryo, 

 it only proves that the time when our ancestors had the 

 fish form is not yet very remote, from the geological 

 point of view. 1 



Finally, natural selection will modify the ontogeny by 

 means of new structures arising. Thus in many ova 

 and larvae special contrivances have been selected ; and 

 as the embryonic development proceeds it must be 

 affected still more owing to the particular method of 

 receiving nourishment and the special position. The 

 insect pupa is a new structure of this character ; the 

 insects cannot possibly have had pupa-like ancestors, 

 as they would never have been able to nourish them- 

 selves. We saw in the preceding chapter how the 

 formation of the pupa came about. 



All these divergences, modifications, and new forma- 

 tions, of which there is an immense number, must alter 



1 For a brief account of the line of man's ancestry the reader may 

 consult Haeckel's " Last Words on Evolution," of which an English 

 translation has just appeared. (A. Owen & Co.) [Trans.] 



