10 M, straus-durckheim's 



we see, then, a new set of organs employed for this function, 

 although it often fulfils another. Thus it is, that in Limulus, 

 where the whole head, and consequently the organs of the 

 mouth, have disappeared, mastication is performed by the feet, 

 which, at the same time, serve as organs of progressive 

 motion. 



Sixth Law. — When one organ governs one, or many 

 others, these follow the march of the dominant organ ; and 

 when at length this disappears, those which were subordinate 

 to it retake suddenly their primitive form. 



It is principally by this law, that we can recognize whether 

 an organ, which is wanting in a species, has disappeared 

 by the effect of its degradation, or is wanting by a merely 

 specific imperfection {simple avortement specifique). In this 

 last case, all that was exclusively subordinate to that organ 

 remains in exactly the same state as if it still existed; and 

 more especially if the subordinate organs have already under- 

 gone a considerable transformation by the effects of the law of 

 variation to which they were submitted, whilst the dominant 

 organ was acting upon them, and that they would be obliged 

 to undergo too great a change to return to their primitive form. 



This law is shown most remarkably in the influence of the 

 wings on the thorax. In proportion as the two pairs of wings 

 change in form and size, the two segments of the thorax which 

 support them follow the same progression, and become more 

 and more united to one another ; but no sooner do the wings 

 disappear in the Aphaniptera, than the two segments of the 

 thorax regain their primitive form, are separated from one 

 another, and again present the same appearance as in Lepisma. 



The wings being first introduced amongst the Coleoptera, 

 and the thorax of these as yet differing but little from that of 

 the Thysanura, it can easily return to the form it possessed pre- 

 viously to its undergoing any modification. It follows thence, 

 that, in those species which are deprived of wings, the thorax 

 returns more or less to its primitive form. This is particularly 

 remarkable in the females of many species of Lampyris, which 

 possess neither wings nor elytra: and this return is moreover 

 occasioned in the Coleoptera, without wings, by another cause, 

 which acts, in this case, on that part of the body ; it is the 

 diminution of the solidity of the integuments in those parts 

 which are covered by the elytra. 



