OTHER THEORIES OF SPECIES-FORMING 273 



of man, of the breast-gland of modem women, etc. The fact that 

 degeneration generally progresses so slowly, often taking thousands 

 and thousands of years, seemed to him a sufficient proof of the inade- 

 quacy of the Li marckian explanation. For if the effect of disuse were 

 transmitted in accumulating ratio in the successive generations, a 

 useless organ ought to disappear much more quickly. 



"Weismann originally attributed a great effect to panmixia, and 

 considered that nearly 90 per cent, of the reduction of rudimentary 

 organs was due to it; the remainder, up to the complete loss of the 

 organs, being accounted for by reversed selection. Romanes was 

 much more modest in his estimate, and only allowed about 10 to 20 

 per cent, to this cause; while Lloyd Morgan gave only 5 per cent, 

 reduction of the original size. The final reduction of the organ to 

 zero is still not accounted for by any of these theories. Calling to aid 

 a failure of the force of heredity, as Romanes did, can hardly be con- 

 sidered a solution of the problem. First of all, the force of heredity 

 does not explain anything in the case. It only restates the problem. 

 We want to know what the force of heredity is. Secondly, if the force 

 of heredity does fail, we should have to explain why it wanes in some 

 cases and not in others. For the reduction and elimination of rudimen- 

 tary organs occurs apparently in the most irregular, haphazard manner. 



" But can panmixia really reduce an organ ? Plate, in agreement 

 with Spencer, Eimer, and others, denies any such possibility. An 

 organ in a given condition of its existence varies around a mean or 

 average, the plus and minus variations generally being equally fre- 

 quent. It follows, therefore, that if all the existing variations are 

 crossed in propagation, the organ remains stationary. Selection only 

 improves the organ by cutting off the minus variations; the absence 

 of selection would simply leave the organ where it was before the 

 selection. At most it could only sink a very Uttle below the aver- 

 age. That this is so is seen in organs which are not under the sway of 

 selection at all. There are numberless such indiJBferent species charac- 

 ters, which ought gradually to dwindle and disappear, yet they remain 

 fairly constant, though continually exposed to the swamping effect 

 of panmixia. Panmixia may explain the functional degeneration of 

 an organ, but cannot explain its actual rudimentation. • 



"Weismann himseK in later times abandoned panmbda as a suffi- 

 cient means of explanation, and resorted to a new theory — that of 

 germinal selection."' 



' From S. Herbert, First Principles of Evolution (1913). 



