390 MB. G, J. EOMANES ON PHYSIOLOGICAL SELECTION. 



offspring had varied suflBciently to become new species, they 

 would still enjoy their ancestral advantages in the struggle for 

 existence. And this, I doubt not, is in part a true explanation; 

 but I also think that the reason why dominant genera are rich 

 in species is chiefly because they everywhere present a great 

 number of individuals exposed to relatively great differences in 

 their conditions of life, or, in other words, that they furnish the 

 best raw material for the manufacture of species by physiological 

 selection, as explained in the last paragraph. Eor, if the fact of 

 dominant genera being rich in species is to be explained only by 

 natural selection, it appears to me that the useful qualities which 

 have already led to the dominance of the ancestral type ought 

 rather to have proved inimical to its splitting up into a number 

 of subordiaate types. If already so far " in harmony with its 

 environment " as to have become for this reason dominant, one 

 would suppose that there is all the more reason for its not under- 

 going change by the process of natural selection. Or, at least, I 

 do not see why the fact of its being in an unusual degree of har- 

 mony with its environment should in itself constitute any unusual 

 reason for its modification by survival of the fittest. On the 

 other hand, as just observed, I do very plainly see why such a 

 reason is furnished for the modifying influence of physiological 

 selection. 



Let us next turn to another of Mr. Darwin's general rules 

 with reference to distribution. He took a great deal of trouble 

 to collect evidence on the two following facts, namely : 1st, that 

 " species of the larger genera in each country vary more frequently 

 than the species of the smaller genera" ; and 2nd, that "many 

 of the species included within the larger genera resemble 

 varieties in being very closely, but unequally, related to each 

 other, and in having restricted ranges."* By larger genera he 

 means genera containing many species ; and he accounts for 

 these general facts by the principle " that where many species of 

 a genus have been formed, on an average many are still forming." 

 But how forming ? If we saj'- by natural selection alone, we 

 should expect to find the multitudinous species differing from one 

 another in respect of features presenting utilitarian significance ; 

 yet this is precisely what we do not find. For Mr. Darwin's 

 argument here consists in showing that " in large genera the 

 amount of difference between the species is often exceedingly 

 * ' Origin of Species,' ed. 6. pp. 44, 4.5, 



