MR. a. J. ROMANES ON PHYSIOLOGICAL SELECTION. 395 



tion thus became the constant distinction, simply because it was 

 in virtue of this distinction, or in virtue of the variation which 

 first originated this distinction, that the species became species ; 

 and the secondary distinctions thus became multitudinous, 

 minute, and unmeaning, simply because they were of later 

 origin, — the result of spontaneous variability, unchecked by inter- 

 crossing with the parent forms, and, on account of their trivial 

 {i. e. physiologically harmless) nature, unchecked also by natural 

 selection, economy of growth, or any other principle which might 

 have prevented spontaneous variability of any other kind. 



EeLATIONS between SuRYIVAL OF THE FlTTEST 



AND Segregation oe the Fit. 



In several preceding parts of this paper, I have had occasion 

 to notice some of the relations between the two forms of selection, 

 natural and physiological. But it seems desirable to consider 

 this matter a little more closely. 



First of all, it will have been observed that the theory of 

 physiological selection in some respects resembles and in other 

 respects differs from that of natural section. Thus to some 

 extent the two theories resemble one another in the kind of 

 evidence by which they are each supported. In neither case does 

 the theory rest upon any actual observation of the origin of 

 species by the agency supposed ; in both cases, therefore, the 

 evidence of the agency is deduced from general considerations 

 regarding the morphology and distribution of specific forms, as 

 well as the observable relations in which such forms now stand to 

 one another. Thus, in the case of each theory alike, the argu- 

 ment takes the form of first establishing a 'prima facie case, 

 showing the antecedent probability of the cause in question ; and 

 next in proving, by a general survey of organic nature, that 

 many of the facts are such as they ought to be if the theory in 

 question is true. 



So far, then, the two theories are logically similar in form ; but 

 in certain material points they widely differ. 



To begin with, it is obvious that as natural selection is a theory 

 of the origin of adaptations, it is a theory of the origin of genera, 

 families, orders, and classes, quite as much as it is a theory of the 

 origin of species. Indeed, as I have already given reasons to 

 show, it appears to me that natural selection is much more a 



