INTRODUCTION 2 1 



ancestral conditions, and not by any means all its 

 ancestors are recapitulated in an animal's develop- 

 ment. Embryos which live in special situations, like 

 the bird embryo developing inside its shell, or the 

 mammal in the womb of its mother, form peculiar 

 organs suited to their particular conditions, and 

 these often have little to do with any ancestral 

 forms. We shall meet other examples of Haeckel's 

 law later on, when discussing different types of 

 larvae. 



Haeckel's law is not strictly an explanation 

 of anything. When a human embryo is developing, 

 its remote fish-like ancestors are long dead and 

 rotted to mud on the sea floor, and cannot possibly 

 be the effective agents which cause the human to 

 form embryonic gill slits. In order to give a satis- 

 factory account of the direct causes of development, 

 one must be able to show how the development is 

 dependent on factors which are actually present in 

 the fertilized egg or its immediate surroundings. 

 What Haeckel did was to find a good way of 

 describing the plethora of odd facts which had been 

 accumulated. We ought to be able to find a reason 

 why so many facts fit into Haeckel's generalization. 

 Actually we still have not found any satisfactory 

 reason, although we can suggest various processes 

 which may be involved. Thus if there is to be an 

 evolutionary change from fish into men, it is 

 obviously easier, or so a man would think, to stick 

 to the old plan of development until it is no longer 

 a help and simply must be altered. The same thing 



