Chap. XXIV. ARRESTED DEVELOPMENT AND RUDIMENTS. 317 



has been partly effected by selection, as with the rudimentary combs and 

 wattles of certain fowls. We have also seen that the wings of some 

 domesticated birds have been slightly reduced by disuse, and the great 

 reduction of the wings in certain silk-moths, with mere rudiments left 

 has probably been aided by disuse. J ' 



With species in a state of nature, rudimentary organs are so extremely 

 common that scarcely one can be named which is wholly free from a 

 blemish of this nature. Such organs are generally variable, as several 

 naturalists have observed; for, being useless, they are not regulated by 

 natural selection, and they are more or less liable to reversion. The same 

 rule certainly holds good with parts which have become rudimentary 

 under domestication. We do not know through what steps under nature 

 rudimentary organs have passed in being reduced to their present con- 

 dition; but we so incessantly see in species of the same group the finest 

 gradations between an organ in a rudimentary and perfect state, that we 

 are led to believe that the passage must have been extremely gradual. It 

 may be doubted whether a change of structure so abrupt as the sudden 

 loss of an organ would ever be of service to a species in a state of nature ; 

 for the conditions to which all organisms are closely adapted usually 

 change very slowly. Even if an organ did suddenly disappear in some 

 one individual by an arrest of development, intercrossing with the other 

 individuals of the same species would cause it to reappear in a more or less 

 perfect manner, so that its final reduction could only be effected by the 

 slow process of continued disuse or natural selection. It is much more 

 probable that, from changed habits of life, organs first become of less and 

 less use, and ultimately superfluous; or their place may be supplied by 

 some other organ; and then disuse, acting on the offspring through 

 inheritance at corresponding periods of life, would go on reducing the 

 organ; but as most organs could be of no use at an early embryonic 

 period, they would not be affected by disuse; consequently they would be 

 preserved at this stage of growth, and would remain as rudiments. In 

 addition to the effects of disuse, the principle of economy of growth, 

 already alluded to in this chapter, would lead to the still further reduction 

 of all superfluous parts. With respect to the final and total suppression 

 or abortion of any organ, another and distinct principle, which will be 

 discussed in the chapter on pangenesis, probably takes a share in the 

 work. 



With animals and plants reared by man there is no severe or recurrent 

 struggle for existence, and the principle of economy will not come into 

 action. So far, indeed, is this from being the case, that in some instances 

 organs, which are naturally rudimentary in the parent-species, become 

 partially redeveloped in the domesticated descendants. Thus cows, like 

 most other ruminants, properly have four active and two rudimentary 

 mammas; but in our domesticated animals, the latter occasionally become 

 considerably developed and yield milk. The atrophied mammas, which, 

 m male domesticated animals, including man, have in some rare cases 

 grown to full size and secreted milk, perhaps offer an analogous case. 

 The hind feet of dogs include rudiments of a fifth toe, and in certain large 

 breeds these toes, though still rudimentary, become considerably developed 



