CRUSTACEA AND MOLLUSCS 249 



And this is, in the present case also, the succession of 

 ancestral stages. 



When an adult animal is modified by natural selection, 

 this modification will be the last to appear in the 

 development of its descendants. It is always the 

 terminal stages of animals that are vitally affected by 

 selection. But the further the modification of the new 

 species proceeds, the more will the embryonic develop- 

 ment be affected. The organs that assume an increasing 

 importance and range in the transformation will no 

 longer find time for their development in the final 

 embryonic stages, and their formation will be pushed 

 further and further back, since those animals will always 

 have the advantage in which the organ is formed first, 

 and so most completely developed. Thus the most 

 important organ in man is the brain. This enormously 

 complicated structure naturally requires a very long time 

 for its construction, and so we can understand why it 

 should be one of the first organs to appear in the 

 embryonic development, and why in the human embryo 

 its size is altogether out of proportion to the small body. 

 But it is utterly wrong to conclude from this that the 

 human embryo with its large head and small body 

 must prove according to the biogenetic law that man's 

 ancestors were similarly misshapen. 



In the transformation of species many new organs 

 become larger and more important than the old ones, 

 and these will accordingly, if they continue to be 

 necessary but can be quickly developed, only be formed 

 at the close of the embryonic life. This in itself will so 

 modify the individual development that it will be almost 



