A. Hyatt— Larval Theory of the Origin of 2 issue. 847 



frequently so similar to purely homoplastic characteristics, that 

 it is not safe to consider any characteristics occurring in distinct 

 groups as homogenous until their phylogenesis has been traced 

 or their comparative embryology is fully understood. 



The hypothesis of the common, but independent origin of 

 types is also supported by all collateral evidences. The results 

 of palasontologic research have carried back the origin of dis- 

 tinct types farther and farther every year. It is now estab- 

 lished, that there was an excessively sudden appearance of vast 

 numbers of forms in the Cambrian or perhaps earlier as claimed 

 by Professor Marcou and others. 



We have applied this specific statement as a generalization 

 to the history of smaller groups of fossils in several branches 

 of the animal kingdom and in many formations, and have 

 found that the sudden appearance of the smaller groups occurs 

 according to the same law. 



There is an obvious plasticity in the animals which first make 

 their appearance in any unoccupied field, or at the beginning of 

 any new formation, which reminds one of the plastic nature, of 

 the most generalized type of Metazoa, the existing Porifera. 

 The generalized types, which alwaj r s occur first in time, ex- 

 hibit like sponges exceptional capacity for adaptation to the 

 most varied requirements of the surroundings, and all of the 

 conditions of the new period or habitat by the rapid develop- 

 ment of numbers of suitable and more highly specialized 

 forms, species and genera. 



The whole picture as presented by morphology, embryology 

 and palaeontology favors the hypothesis we have previously 

 advanced in other papers, namely, that the early geologic his- 

 tory of animal life, like the early stages of development in the 

 embryo was a more highly concentrated and accelerated process 

 in evolution than that which occurred at any subsequent 

 period of the earth's history. 



The history of the Porifera and higher Protozoa suggests 

 also that the evolution oE the Metazoa may have occurred 

 more rapidly than we can now calculate. One of the great 

 errors of the present day is the assumption that such changes 

 and transitions occurred slowly and gradually; and it is evi- 

 dent that this assumption is based almost wholly upon inves- 

 tigation of the more highly specialized animals in which the 

 capacity for change may be reasonably considered as very 

 much less than in their more generalized and embryonic ances- 

 tral forms. 



