22 HOW ANIMALS DEVELOP 



happens in human inventions. When motor-cars 

 were first made, the engineers did not think the 

 whole problem out from the beginning and produce 

 a stream-lined model with the engine over the back 

 wheels where the power is required: they seem to 

 have been exhausted by thinking out the engine, 

 and simply attached it to the current form of horse- 

 carriage in front where the horse had been. We can 

 call this a sort of "habit reason" ; men inventing, 

 and embryos developing, tend to do what their 

 fathers did if they can, because it is easier. 



There may be other and more important reasons 

 for imitating an old pattern. It may act as a guide. 

 For instance, if one is going to make a cast-iron pot, 

 one first models it in clay, as though iron had not 

 yet been discovered; from the clay pot a mould is 

 made into which the molten iron can be poured. 

 Here the "ancestral" clay pot provides what may 

 be called a formative stimulus for the "more highly 

 evolved" iron pot. Perhaps this sort of explanation 

 applies to ancestral characters which appear in 

 embryos only for a short time, eventually dis- 

 appearing entirely. 



The Three Fundamental Layers 



It follows from Haeckel's biogenetic law that young 

 embryos must look much alike, since they should 

 show only the characters which are common to all 

 types of animals. This deduction from the law is 

 actually true. Even before Haeckel definitely formu- 

 lated his law quite a large number of different kinds 



