Appendix I. 305 



on developing the foot-stalks than the eyes which were 

 mounted upon them so that while the latter were left by 

 selection with " a strong tendency to deterioration," the 

 former have had this tendency " bred out in the race " l ? 



To sum up. There is now no question in any quarter 

 touching the fact that panmixia, or the cessation of selection, 

 is a true cause of degeneration. The only question is as to 

 the amount of degeneration which it is able to effect when 

 not assisted by the reversal of selection, or any other 

 cause of degeneration. Moreover, even with regard to this 



1 Of course it must be observed that degeneration of complexity 

 involves also degeneration of size, so that a more correct statement 

 of the case would be Why, under the cessation of selection, does an 

 organ of extreme complexity degenerate much more rapidly than one of 

 much less complexity? For example, under domestication the brains 

 of rabbits and ducks appear to have been reduced in some cases by 

 as much as 50 per cent. (Darwin, and Sir J. Crichton Browne.) But 

 if it is possible to attribute this effect or part of it to an artificial 

 selection of stupid animals, I give in the text an example occurring 

 under nature. Many other cases, however, might be given to show the 

 general rule, that under cessation of selection complexity of structure 

 degenerates more rapidly and also more thoroughly than size of it. 

 This, of course, is what Mr. Gallon and I should expect, seeing that the 

 more complex a structure the greater are the number of points for 

 deterioration to invade when the structure is no longer "protected by 

 selection." (On the other hand, of course, this fact is opposed to the 

 view that degenerat ion of useless structures below the " birth-mean " of 

 the first generations, is exclusively due to the reversal of selection ; for 

 economy of growth, deleterious effect of weight, and so forth, ought to 

 affect size of structure much more than complexity of it.) But I choose 

 the above case, partly because Professor Lloyd Morgan has himself 

 alluded to " the eyes of Crustacea," and partly because Professor Ray 

 Lankester has maintained that the loss of these eyes in dark caves is due 

 to the reversal of selection, as distinguished from the cessation of it. In 

 view of the above parenthesis it will be seen that the point is not of 

 much importance in the present connexion ; but it appears to me that 

 cessation of selection must here have had at least the larger share in the 

 process of atrophy. For while the economy of nutrition ought to have 

 removed the relatively \z.\% foot-stalks as rapidly as the eyes, I cannot 

 see that there is any advantage, other than the economy of nutrition, to 

 be gained by the rapid loss ot hard- coated eyes t even though they have 

 ceased to be of use. 



II. X 



