THE CAUSATION OF DISEASE. I I J . 



happens, however, that the evolution displayed in the history of 

 a machine fulfils the terms of Spencers formula, many machines 

 being improved by successive additions, whereby the structure 

 becomes more " integrated '"' and more complex (heterogeneous). 



Let us now suppose that some particular machine has been 

 thus very much improved by some slight addition to its struc- 

 ture. It will contain potentially the ability to work in the 

 old-fashioned manner ; for, if the recent addition be removed, 

 a partial dissolution being thus effected, this potentiality will 

 be manifested. 



2. The incomplete local evolutions which take place in 

 disease afford another example of the conversion oi potentialities 

 into actualities, for it is obvious that, in this process, the 

 affected tissues take on characters which they did not possess 

 before. 



Both in partial evolution and partial dissolution the pro- 

 cess, as we have seen, is often, indeed generally, vitiated ; 

 and it therefore by no means follows that when the poten- 

 tiality becomes manifested by either of these methods, such 

 potentiality has ever existed as a normal actuality in the 

 ancestral history of the individual ; for, owing to the action of 

 E upon S, the S will cause, so to speak, natural variations in 

 the partially evolved or dissolved tissue. Nevertheless, such 

 structural characters must have existed potentially in the 

 tissues. Thus, the tissues contain myriads of potentialities, 

 many of which never have been nor ever will be manifested, for, 

 to suppose that every potentiality has been manifested, is to 

 suppose that the tissues have been subjected to every con- 

 ceivable form of E, seeing that every different form of E is 

 capable of affecting the tissues in a different way. 



3. In both the methods already given, the potentiality 

 becomes an actuality by the local falling away in the evo- 

 lutionary value of a mature tissue. An actuality, before 

 only latent, may, however, also be developed by an advance in 

 evolution. Thus, an embryo contains all the potentialities of 

 the future individual. Of this method little need be said. 

 Many such potentialities are displayed in the natural and, so 

 to speak, unaided process of evolution, and many may be 

 developed by a special education ; indeed, it often happens that 



