So THE CAUSATION OF DISEASE. 



are able to keep in equilibrium with the forces of the outer 

 world as represented by E. When such perfect equilibrium 

 obtains, the organism is said to be perfectly adapted to its E. 

 And if any marked alteration in the E occurs, this has to be 

 met by a modification in the distribution of the bodily forces, 

 by a process of adaptation, as it is called. Now, although 

 each individual parent may be in perfect equilibrium with his 

 or her respective E, it is quite possible that the mean struc- 

 tural product of the two shall be in no such perfect equili- 

 brium with its E. Herbert Spencer, so far as I know, does not 

 consider the possibility of disease thus resulting, being content 

 to deal with generalities. Nevertheless, there is no doubt 

 that some forms of disease may thus result. Let us see how. 



It is a well-known fact that crossing is a cause of varia- 

 bility.* And, as might be expected, the variations thus 

 resulting may be ill adapted to the E, and in this way pre- 

 dispose to disease ; in some cases they may appear as actual 

 disease, for, as we shall presently see, these new characters 

 may be simple reversions, and such reversions, though nor- 

 mal in the remote ancestors from whom they are derived, 

 may be entirely abnormal in their remote progeny. A 

 branchial cleft, for example, though normal in a fish, would 

 be totally abnormal during the post-partem life of a mammal. 

 The crossing referred to is one between distinct varieties ; but 

 inasmuch as all human beings differ from one another, and 

 some far more than others, it may occasionally happen that 

 father and mother exhibit, in some particular, a great differ- 

 ence, so that their union may be, in respect to the particulars 

 wherein they differ, more or less a cross, leading to a reversion, 

 perhaps not to a very remote ancestral state, but to a condition 

 of body entirely out of harmony with the existing civilized E. 



But setting aside the question of reversion, it is quite possible 

 that a considerable variation may arise from such a union, for it 

 appears certain that " quite new characters" may result from 

 crossing. " The common opinion of floriculturists proves that the 

 crossing and recrossing of distinct but allied plants .... induces 

 excessive variability, having the appearance of quite new charac- 

 ters " (that is, not mere reversions). Now, are we justified in 

 * Vide Darwin, " Variation under Domestication," pp. 252-255. 



