THE EVOLUTION OE BIEDS. 



are beneficial, and is confessedly inapplicable to variations that are not so. But the question 

 arises, Do variations survive that are not beneficial ? Upon this point authorities differ. 

 Wallace argues that the fact of the survival of a variation is proof positive that it was the 

 fittest variation offered to the choice of Natural Selection at the time ; and that, if we fail to 

 perceive that such variation was beneficial, the fault is not in the variation but in our imperfect 

 powers of perception. This is a very plausible theory, so plausible, indeed, that it may be 

 regarded as a probable hypothesis ; but unfortunately its acceptance does not remove the great 

 difficulty of the swamping effects of interbreeding, by which isolated variations, however 

 beneficial, must inevitably disappear after a few generations. To this Wallace replies, 

 that variations are not isolated, that no fact in natural history is better established than the 

 fact that variations occur simultaneously, in a great number of individuals, in the same 

 area. I have not the least doubt that Wallace is perfectly right. If it were not so, the 

 theory of Evolution must inevitably fall to the ground. But it seems to me that, by the 

 admission of this fact, Wallace has dethroned his theory of Natural Selection from its 

 proud position as the main factor in the Origin of Species. Variations which occur 

 simultaneously, in the same area, in a great number of individuals, and for a great 

 number of generations, cannot possibly be fortuitous ; but if they be definite, they are 

 quite capable of accounting for all the facts of Evolution, in districts where there is no 

 struggle for existence, without the aid of selection of any kind. If once we admit that 

 variation is not fortuitous, Natural Selection can no longer be regarded as the cause of 

 Evolution, but only its guide. 



Natural Selection cannot produce Evolution ; it only quickens its pace, and prevents it 

 from going astray. The differentiation of species can only take place in Isolation, either 

 of time or space. The causes of Evolution are the causes of Variation, some of which we 

 know, but the most important of which we have not yet discovered. It is, however, 

 necessary that the question should be studied from the Darwinian point of view. It will 

 scarcely be denied that, as an exponent of Evolution, Darwin stands head and shoulders 

 above all his rivals. Darwin disposed of the difficulty of apparently useless variations in 

 two ways : many characters are confined to the male, many are only assumed when the 

 male becomes adult, and many are only temporarily assumed, being laid aside when the 

 breeding-season is over, to be reassumed the following spring. 



These characters are ascribed by Darwin to the influence of Sexual Selection. 



Other characters, common to both sexes, are ascribed to correlation, the apparently 

 useless character being, for some inexplicable reason, generally associated with an obviously 

 useful character, the frequency of the coincidence leading to the conclusion that the two 

 characters are really, and not only apparently, connected. 



The law of correlation adds nothing to our knowledge, unless the discovery that a 

 cause exists, of which we are ignorant, may be regarded as such. 



If Wallace be right in his theory that all variation which survives must of necessity 

 be beneficial, irrespective of our ability to discover the benefit, then of course the theory of 



All survi- 

 ving varia- 

 tions bene- 

 ficial. 



Variations 

 seldom iso- 

 lated. 



Therefore 

 not acci- 

 dental. 



Isolation 

 necessary. 



Sexual 

 selection. 



Correlation : 

 a term used 

 to express 

 ignorance of 

 the true 

 cause. 



