- 



352 



LAWS OF VAKIATIOX. Chap. XXVI. 



now undergoing rapid improvement, those parts or characters 

 which are the most valued vary the most. This naturally fol- 

 lows from recently selected characters continually tending to 

 revert to their former less improved standard, and from their 

 being still acted on by the same agencies, whatever these may 

 be, which first caused the characters in question to vary. The 

 same principle is applicable to natural species, for, as stated 

 in my ' Origin of Species,' generic characters are less variable 

 than specific characters ; and the latter are those which have 

 been modified by variation and natural selection, since the period 

 when all the species belonging to the same genus branched off 



j, V^ K^ ^mv, & 



from a common progenitor, whilst generic characters are those 

 which have remained unaltered from a much more remote epoch, 

 and accordingly are now less variable. This statement makes 

 a near approach to Mr. Walsh's law of Equable Variability. 

 Secondary sexual characters, it may be added, rarely serve to 

 characterise distinct genera, for they usually differ much in the 

 species of the same genus, and are highly variable in the indi- 

 viduals of the same species ; we have also seen in the earlier 

 chapters of this work how variable secondary sexual characters 

 become under domestication. 



■» 



Summary of the three previous Chapters, on the Laivs of Variat 



In the twenty-third chapter we have seen that changed 

 ditions occasionally act in a definite manner on the orgi 

 so that all, or nearly all, the individuals thus exposed become 

 modified in the same manner. But a far more frequent result 

 of changed conditions, whether acting directly on the organisa- 

 tion or indirectly through the reproductive system being affected 

 is indefinite and fluctuating variability. In the three latter 

 chapters we have endeavoured to trace some of the laws by 

 which such variability is regulated. 



Increased use adds to the size of a muscle, together with the J 



blood-vessels, nerves, ligaments, the crests of bone to which these 

 are attached, the whole bone and other connected bones. So it 

 is with various glands. Increased functional activity strengthens 

 the sense-organs. Increased and intermittent pressure thickens 

 the epidermis ; and a change in the nature of the food some- 

 times modifies the coats of the stomach, and increases or 



