344 LAWS OF VAKIATION. C^av XXVI 



I have discussed this subject of analogous variation at con- 

 Biderable length, because it is well known that the varieties 

 of one species frequently resemble distinct species — a fact in 

 perfect harmony with the foregoing cases, and explicable on 

 the theory of descent. Secondly, because these facts are 

 important from showing, as remarked in a former chapter, 

 that each trifling variation is governed by law, and is deter- 

 mined in a much higher degree by the nature of the organi- 

 sation, than by the nature of the conditions to which the 

 varying being has been exposed. Thirdly, because these facts 

 are to a certain extent related to a more general law, namely, 

 that which Mr. B. D. Walsh ^o has called the "Law of 

 Equable Variability" or, as he explains it, " if any given cha- 

 " racter is very variable in one species of a group, it will tend 

 " to be variable in allied species ; and if any given character 

 " is perfectly constant in one species of a group, it will tend 

 " to be constant in allied species." 



This leads me to recall a discussion in the chapter on 

 Selection, in which it was shown that with domestic races, 

 which are now undergoing rapid improvement, those parts 

 or characters vary the most, which are the most valued. 

 This naturally follows from recently selected characters con- 

 tinually 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 cha- 

 racters 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 varia- 

 tion and natural selection, since the period when all the 

 species belonging to the genus branched oif from a common 

 progenitor, whilst generic characters are those which have 

 remained unaltered from a much more remote epoch, and ac- 

 cordingly 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 difi'er much in 

 the species of the same genus, and they are highly variable 



^0 ' Proc. Entomolos:. Soc. of Philadelphia,' Oct. 1863, p. 213. 



