300 SIGXIFICANCE OF SEXUAL REPRODUCTION [V. 



of the species will gain what the functionless organ loses in 

 size and nutrition. As at each stage of retrogressive trans- 

 formation individual fluctuations always occur, a continued 

 decline from the original degree of development will inevitably, 

 although very slowly, take place, until the last remnant finally 

 disappears. How inconceivably slowly this process goes on 

 is shown by the numerous cases of rudimentary organs : by 

 the above-mentioned embryonic sixth finger of man, or by 

 the hind hmbs of whales buried beneath the surface of the 

 body, or by their embr3^onic tooth-germs. I believe that the 

 very slowness with which functionless organs graduall}' disap- 

 pear, agrees much better with my theory than with the one 

 which has been hitherto held. The result of the disuse of an 

 organ is considerable, even in the course of a single individual 

 life, and if only a small fraction of such a result were trans- 

 mitted to the descendants, the organ would be necessarily 

 reduced to a minimum, in a hundred or at any rate in a 

 thousand generations. But how many millions of generations 

 may have elapsed since e.g. the teeth of the whalebone whales 

 became useless, and were replaced by whalebone! We do 

 not know the actual number of years, but we know that the 

 whole material of the tertiary rocks has been derived from the 

 older strata, deposited in the sea, elevated, and has been itself 

 largely removed by denudation, since that time. 



Now if this theory as to the causes of deterioration in disused 

 organs be correct, it follows that rudimentary organs can only 

 occur in species with sexual reproduction, and that they cannot 

 be formed in species which are exclusively reproduced by the 

 parthenogenctic method : for, according to my theory, varia- 

 bility depends upon sexual reproduction, while the deteriora- 

 tion of an organ when disused, no less than its improvement 

 when in use, depends upon variability. There are therefore 

 two reasons which lead us to expect that organs which are no 

 longer used will remain unreduced in species with asexual 

 reproduction : first, because only a very slight degree of 

 hereditary variability can be present, viz. such a degree as 

 was transmitted from the time when sexual reproduction was 

 first abandoned by the ancestors ; and, secondly, because even 

 these slight degrees of variability are not combined, or, in other 

 words, because panmixia cannot occur. 



