138 Dr Barry on the Unity of Structure 



direction taken in development ; determines, therefore, the struc- 

 ture of the new being. 



The general direction taken in the development of all the in- 

 dividuals of a species, is the same ; but there is a particular 

 direction, proper to the development of each individual, and 

 therefore a particular structure, not identical with any other ; 

 for in no two individuals, is the sum of the innate (plastic) pro- 

 perties in all respects the same. 



It has been already said, that as the elements of an individual 

 cease, in turn, to be constituent parts of the same, the identity 

 of that individual must be continually changing can exist, in- 

 deed, at no two periods of time ; inasmuch as new elements are 

 continually entering into its constitution, while old ones are de- 

 parting. 



Hence, individual peculiarities in structure must, in their 

 turn, become hereditary to succeeding sets of elements ; con- 

 tinually renewed, as we have just asserted these elements to be. 

 There must, besides, continually present themselves, fresh pe- 

 culiarities; and in their turn, these also must be inherited by sets 

 of elements succeeding. 



For the same reasons, the first set of elements, constituting a 

 germ, proceeding, as already said, from the elements of a parent 

 or of parents, must possess properties that were characteristic of 

 the latter, at the moment when their separation took place ; and 

 can indeed possess no othe/s, since the elements of the parents, 

 and therefore the properties, are continually changing. 



Hence it is, that the sum of the innate (plastic) properties can 

 be in no two individuals the same ; hence the particular direction 

 of development proper to each individual ; * and hence indivi- 

 dual peculiarities of structure. 



Strictly speaking, therefore, no two individuals of different 

 births can have the same parentage ; for though the individuality 

 of the parent, or of each parent, does not change, yet, as indivi- 

 duals, the parents are continually changing. 



The more nearly cotemporaneous separation of their elements, 

 and the cotemporaneous derivation of nourishment from the 

 maternal fluids, during foetal life, but especially the former, 

 are perhaps the causes why twins are sometimes so much alike in 



One general direction, as said before, being common to a species. 



