v.] IN THE THEORY OF NATURAL SELECTION. 299 



this explanation, we shall have to start from the supposition 

 that new forms are not only created by natural selection, but 

 are also preserved by its means. In order that any part of the 

 body of an individual of any species may be kept at the maxi- 

 mum degree of development, it is necessary that all individuals 

 possessing it in a less perfect form must be prevented from 

 propagation — they must succumb in the struggle for existence. 

 I will illustrate this by a special instance. In species which, 

 like the birds of prey\ depend for food upon the acuteness of 

 their vision, all individuals with relatively weak eyesight must 

 be exterminated, because they will fail in the competition for 

 food. Such birds will perish before they have reproduced 

 themselves, and their imperfect vision is not further trans- 

 mitted. In this way the keen eyesight of birds of prey is kept 

 up to its maximum. 



But as soon as an organ becomes useless, the continued 

 selection of individuals in which it is best developed must 

 cease, and a process which I have termed panmixia takes 

 place. When this 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 ■■^. This result is all the more likely to happen, 

 inasmuch as other organs which are of importance for the life 



1 I here make use of the same illustration which I emploj-ed in my 

 first attempt to explain the effects of panmixia. Compare the second 

 Essay, ' On Heredity.' 



^ [E. Ray Lankester has suggested (Encycl. Britann., art. 'Zoology,' 

 pp. 818, 819) that the blindness of cave-dwelHng and deep-sea animals 

 is also due to the fact that ' those individuals with perfect e^'es 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 panmixia alone. — E. B. P.J 



