SECT, xiv.] INTRODUCTION. 77 



if this supposition is made, it cannot be made again for the 

 equally perfect forms with three petals, and the rest 1 . 



The hypothesis of Reversion to account for the Symmetry 

 and perfection of modern or discontinuous Variation is made 

 through a total misconception of the nature of Symmetry. 



There is a famous passage in the Descent of Man, in which 

 Darwin argues that the phenomenon of double uterus, from its 

 perfection, must necessarily be a Reversion. 



"In other and rarer cases, two distinct uterine cavities are formed, each 



having its proper orifice and passage. No such stage is passed through during the 

 ordinary development of the embryo, and it is difficult to believe, though perhaps 

 not impossible, that the two simple, minute, primitive tubes could know how (if 

 such an expression may be used) to grow into two distinct uteri, each with a well- 

 constructed orifice and passage, and each furnished with numerous muscles, nerves, 

 glands and vessels, if they had not formerly passed through a similar course of 

 development, as in the case of existing marsupials. No one will pretend that so 

 perfect a structure as the abnormal double uterus in woman could be the result 

 of mere chance. But the principle of reversion, by which long-lost dormant 

 structures are called back into existence, might serve as the guide for the full 

 development of the organ, even after the lapse of an enormous interval of time-." 

 fiescent of Man, vol. i. pp. 123 and 1'24. 



This kind of reasoning has been used by others again and 

 again. It is of course quite inadmissible ; for by identical reason- 

 ing from the perfect symmetry of double monsters, of the single 

 eye of the Cyclopian monster, and so on, it might be shewn that 

 Man is descended from a primitive double vertebrate, from a 

 one-eyed Cyclops and the like. For other reasons it is likely 

 enough that double uterus was a primitive form ; but the per- 

 fection and symmetry of the modern variation to this form is 

 neither proof nor indication of such an origin. Such a belief 

 arises from want of knowledge of the facts of Meristic Variation, 

 and is founded on a wrong conception of the nature of symmetry 

 and of the mechanics of Division. The study of Variation shews 

 that it is a common occurrence for a part which stands in the 

 middle line of a bilaterally symmetrical animal, to divide into 

 two parts, each being an optical image of the other: and that 

 conversely, parts which normally are double, standing as optical 

 images of each other on either side of such a middle line may 



1 For a full account of such facts, see a paper by Miss A. BATESON and myself 

 On Variations in P'loral Symmetry. Journ. Linn. Soc., xxvui. p. 386. 



This extraordinary passage is scarcely worthy of Darwin's penetration. If 

 read in the original connexion it will seem strange that it should have been allowed 

 to stand. For in a note to these reflexions on Reversion (Descent, i. p. 125) Darwin 

 refers to and withdraws his previously expressed view that supernumerary digits 

 and mamrnse were to be regarded as reversions. This view had been based on the 

 perfection and symmetry with which these variations reproduce the structure of 

 putative ancestors. It was withdrawn because Gegenbaur had shewn that poly- 

 dactyle limbs often bear no resemblance to those of possible ancestors, and because 

 extra mammae may not only occur symmetrically and in places where they are 

 normal in other forms, but also in several quite anomalous situations. In the light 

 of this knowledge it is strange that Darwin should have continued to regard the 

 perfection and symmetry of a variation as evidence that it is a Reversion. 



