Appendix I. 299 



for Existence between Parts of an Organism that the principle 

 of selection must operate in every constituent tissue, and as 

 between every constituent cell of which an organism is com- 

 posed. Now, if an organ falls into disuse, its constituent cells 

 become worsted in their struggles with other cells in the 

 organism. Hence, degeneration of the disused organ may 

 progressively increase, quite independently of any struggle 

 for existence on the part of the organism as a whole. Con- 

 sequently, degeneration may proceed without any reference 

 to the principle of " economized nutrition " ; and, if it does 

 so, and if the effects of its doing so are transmitted from 

 generation to generation, the disused organ will finally dis- 

 appear by means of Roux's principle. 



The long communication above quoted led to a still longer 

 correspondence in the pages of Nature. For Professor Ray 

 Lankester wrote 1 to impugn the doctrine of panmixia,or cessa- 

 tion of selection, in Mo, arguing with much insistence that 

 " cessation of selection must be supplemented by economy of 

 growth in order to produce the results attributed to panmixia." 

 In other words, he denied that panmixia alone can cause 

 degeneration in any degree at all : at most, he said, it can 

 be but " a condition," or " a state," which occurs when an 

 organ or part ceases to be useful, and therefore falls under 

 the degenerating influence of active causes, such as economy 

 of nutrition. Or, in yet other words, he refused to recognize 

 that any degenerative process can be due to natural selection 

 as merely withdrawn : only when, besides being withdrawn, 

 natural selection is reversed, did he regard a degenerative 

 process as possible. As a result of the correspondence, 

 however, he eventually * agreed that, if the " birth-mean " of 

 an organ, in respect either of size or complexity of structure, 

 be lower than the " selection-mean" while the organ is useful 

 (a fact which he does not dispute) ; then, if the organ ceases 



1 Nature, vol. xli. p. 486. * Ibid. vol. xlii. p. 52. 



