96 INVERTEBRATE PALAEONTOLOGY 



structures, and almost all arc so minute that preservation 

 and isolation are unlikely. Further, in the rare cases 

 when early developmental forms are discovered, it is 

 usually impossible to determine the relationship between 

 larva and adult, since it is too late to watch the growth 

 of one into the other. Nevertheless, Palaeontology can 

 make use of certain ontogenetic qualities in adult or 

 adolescent organisms that, within restricted limits, give 

 a kind of recapitulation. 



There are two " laws of stages of development " recog- 

 nized, but they differ chiefly in application and mode 

 of expression. One, the " law of stages in ontogeny " 

 (Hyatt), is merely an extension of that of recapitulation. 

 It states that in young forms, stages occur which cor- 

 respond with the adult stages of ancestral types ; while 

 in senile individuals, stages occur which may repeat, 

 in reverse order, those found in the young, and are in 

 some measure prophetic of coming changes in subse- 

 quent evolution of the group concerned. Expressed 

 in a simpler form, the law states that recapitulation 

 continues till maturity, and is followed by anticipation 

 of future changes in the stock. Hence it follows that 

 study of the complete life-history of an individual 

 supplies an abridged epitome of the history of the 

 group to which it belongs. The other law, that of 

 " stages in morphogeny " (usually and originally called 

 the " law of localized stages in development," Jackson), 

 states that throughout the life of an individual, there 

 occur in local structures stages comparable with those 

 found in youth or senility, supplying evidence on racial 

 evolution similar to that afforded by ontogenetic stages. 

 In other words, the growth of separate structures or 

 organs follows recapitulatory or anticipatory courses in 

 some respects independent of the age of the individual. 



Both of these hypotheses are capable of appreciation, 



