292 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 



process is in operation, not only those individuals with the best- 

 developed organs have the chance of reproducing- themselves, but 

 also those individuals in which the organs are less well-developed. 

 Hence follows a mixture of all possible degrees of perfection, 

 which must in the course of time result in the deterioration of the 

 average development of the organ. Thus a species which has retired 

 into dark caverns must necessarily come to gradually possess less 

 developed powers of vision ; for defects in the structure of the 

 eyes, which occur in consequence of individual variability, are not 

 eliminated by natural selection, but may be transmitted and fixed 

 in the descendants 1 . This result is all the more likely to happen, 

 inasmuch as other organs which are of importance for the life 

 of the species will gain what the functionless organ loses in size 

 and nutrition. As at each stage of retrogressive transformation 

 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 limbs of whales buried beneath 

 the surface of the body, or by their embryonic tooth-germs. 

 I believe that the very slowness with which functionless organs 

 gradually disappear, 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 genera- 

 tions. 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, 



[ l E. Ray Lankester has suggested (Encycl. Britann., art. ' Zoology,' pp. 818,819) 

 that the blindness of cave-dwelling and deep-sea animals is also due to the fact that 

 ' those individuals with perfect eyes would follow the glimmer of light and eventually 

 escape to the outer air or the shallower depths, leaving behind those with imperfect 

 eyes to breed in the dark place. A natural selection would thus be effected.' Such 

 a sifting process would certainly greatly quicken the rate of degeneration due to pan- 

 mixia alone. E.B. P.] 



