60 GENERAL ZOOLOGY 



LAW OF BIOGENESIS 



The Crustacea may well serve to illustrate one of the 

 great principles of zoology are particularly appropriate, 

 in fact, because the law in question was first discovered 

 while the classification of this class was being investigated. 

 The "Law of Biogenesis, " otherwise known as the " Re- 

 capitulation Theory," holds that ontogeny repeats phy- 

 logeny i.e., that each individual animal in its develop- 

 ment repeats the stages through which its race has passed 

 in its evolution. Huxley once put the idea briefly by 

 saying: " Every animal climb.s its own ancestral tree." 



This law has been of great value to zoologists in helping 

 to work out the true relationships of animals. For example, 

 because man, in his early embryonic development, has gill- 

 like structures at the sides of his neck and a fish-like form, 

 it is believed that he came from what was once a fish-like 

 ancestor which lived in the water. Furthermore, it seems 

 reasonable to believe that all vertebrates came from a 

 common ancestry, because, up to a certain point, the de- 

 velopmental history of all is very similar. The embryonic 

 record is often dim, or warped. There are instances where 

 stages which should come later are sometimes pushed ahead, 

 and embryonic animals sometimes acquire new specializa- 

 tions which their remote ancestors did not possess, but the 

 great landmarks in the embryonic records generally keep 

 their resemblance to the racial, or phylogenetic, history so 

 that the story is readable. 



The value of the Law of Biogenesis is well illustrated in 

 the Crustacea. The barnacles, for example, were until a 

 short time ago of very questionable relationships. Some 

 zoologists believed that they should be placed with the 

 molluscs because they had a hard shell, others that they 

 should be classed with the worms, but no one thought of 

 relating them to crustaceans. Then it was discovered that 

 many entomostracans hatch from the egg as a little six- 

 legged larva, the nauplius. This nauplius larva swims 



