348 mb. g. j. eomanes on physiological selection. 



Evolution of Species by Independent Yaeiation. 



Enough has now been said to justify the view that there must 

 be some cause or causes, other than natural selection, operating 

 in tbe evolution of species. And this is no more than Mr. 

 Darwin himself has expressly and repeatedly stated to have been 

 his own view of the matter ; nor am I aware that any of his 

 followers have thought otherwise. Hitherto the only additional 

 causes of any importance that have been assigned are use and 

 disuse, sexual selection, correlated variability, and yet another 

 principle which 1 believe to have been of much more importance 

 than any of these — not eveu excepting the first, where the origin 

 of species only is concerned. Yet it has attracted so little 

 attention as scarcely ever to be noticed by writers on Evolution, 

 and never even to have received a name. Eor the sake of con- 

 venience, therefore, I will call this principle the Preventioa of 

 Intercrossing with Parent Eorms, or the Evolution of Species 

 by Independent Variation. 



Eirst, let us consider how enormous must be the number of 

 variations presented by every generation of every species. Ac- 

 cording to the Darwinian theory, it is only those variations which 

 happen to have been useful that have been preserved ; yet, even 

 as thus limited, the principle of variability is held to have been 

 sufl6.cient to furnish material out of which to construct the whole 

 adaptive morphology of nature. How immense, therefore, must 

 be the number of unuseful variations. These are probably many 

 hundred of times more numerous than the useful variations, 

 although they are all, as it were, stillborn, or allowed to die out 

 immediately by intercrossing. Hence, as a matter of fact, we 

 find that no one individual " is like another all in all ;" which is 

 another way of saying that a specific type may be regarded as 



tures are very far from being convertible terms. On the one hand, as we have 

 seen, many useful structures are shared by many species in common ; and, on 

 the other hand, many species-distinguishing structures are not useful. There- 

 fore I say that the theory which explains the origin of useful structures is not, 

 strictly speaking, a theory of the origin of species ; it only explains the origin 

 of species in cases where it happens that one species differs from another in 

 respect of features all of which present utiUtarian significance. And this, as 

 even Mr. Darwin himself allows, is very far from being universally, or even 

 usually, the case. 



