92 THE PRINCIPLES OF HEREDITY 



phenomena of evolution. " If acquired variations are trans- 

 mitted, there must be some unknown principle in heredity ; 

 if they are not transmitted, there must be some unknown 

 factor in evolution." l " Something is wanting to the selection 

 of Darwin and Wallace, which it is obligatory on us to dis- 

 cover if we can, and without which selection as yet offers no 

 complete explanation of the phyletic processes of transforma- 

 tion." 2 But biologists have failed to realize that recapitula- 

 tion, variation, and bi-parental reproduction combined render 

 reversion inevitable, which in turn renders the deterioration 

 and final disappearance of useless organs and variations i. e. 

 organs and variations not preserved by Natural Selection 

 equally inevitable. 



156. This, then, is the unknown factor in evolution. Not 

 one, but two forces are continually at work in nature Natural 

 Selection and Reversion. The former causes progressive 

 evolution, the latter regressive evolution. They are opposed, 

 but one would be quite inadequate without the other. They 

 are warring forces, but their resultant is a near approach to 

 perfection. Between them they tend to bring every species 

 into exquisite harmony with its environment. The parts they 

 play in evolution may be compared to those played by 

 assimilation and waste in the growth of the individual. 

 Not substance alone, but waste of substance, renders an 

 individual efficient. 



157. Some biologists, perceiving the inadequacy of Natural 

 Selection, have attributed regression to transmission of 

 acquirements in this case to the transmission of the effects 

 of disuse. Others, perceiving^ Jbhe 



^ 



Selection, and Jhe ifiij^lSKlity QJJbhejTansmis^i 

 men ts, have_ appealed^jxTj^^ 



Others, forgetting that mere cessation of selection is sufficient 

 to cause regression, have declared that useless parts are 

 injurious, inasmuch as they are burdensome and absorb 

 nutriment, and that, therefore, Natural Selection (in this 

 case Reversed Selection) is an all-sufficient cause. Others 

 have attributed regression to the struggle of parts, 

 alleging that the more active, the more used parts, absorb 

 the most nutriment. But this implies the transmission of 

 acquirements. Moreover, there is no real struggle of parts. 

 Plants and animals are not always on the verge of starvation ; 

 the roe of the herring does not struggle, but it absorbs a 



1 Osborn. 



* Weismann. Germinal Selection, English translation, p. 15. Compare 

 also Romanes' Darwin and After Dartm'n, vol. ii., p. 154. 



