282 STUDIES IN THE EVOLUTION OF ANIMALS 



taken off, or something fused with another, so that traces of an 

 evolution of one type from another which may have preceded it 

 are in most cases evident. 



Then if the Divine thought of Agassiz was not the director 

 of the evolution of these new types, is there any other mode of 

 explaining the sudden appearance of what may truly be called 

 monstrosities ? 



The Science of Teratology has recorded a great number of 

 monstrosities, both in the vegetable and animal kingdoms. What 

 brings them about ? 



There seem to be two divisions of monstrosities : 



{a) Fusion, more or less complete, of tzvo separate individuals 

 (twins), into one compound body, with or without suppression of 

 some of the parts ; and 



{p) Fusion of different parts of the same individual ; separation 

 of originally united parts ; displacement or suppression of various 

 parts ; or re-appearance of some ancestral character, by what is 

 called reversion (atavism) ; or enlargement or diminution of a part. 



We know .that in the fully formed individual the nerve-centres, 

 with their communicating nerves, control everything. Every 

 function of the body is controlled by them. It is reasonable there- 

 fore to infer that, during embryonic development, as soon as the 

 nervous system has had a commencement, it must begin to control 

 the evolution of every part of the body ; and the slightest change, 

 from whatever cause, from the normal that may occur in the nerve- 

 centre, or prime-mover, is likely to be reflected as a modification 

 from the normal at the distal end of the nerve or nerves which 

 that nerve-centre controls. A very slight change in the embryo 

 may result in much more extensive changes in the fully formed 

 animal. 



It would appear a not uncommon thing for the controlHng 



