8 



M. STRAUS-DURCKHEIM S 



Second Law. — In those organs which form series on the 

 same animal, the gradation is commonly double : that is to say, 

 on the one hand it is relative to the succession of genera and 

 families ; on the other, it is relative to the rank which each pair 

 occupies on the body of the animal. 



This is exemplified in the feet; the rudiments of which 

 appear in Lombricus without any articulation, as mere cirrhi, 

 and buried in the skin. These soon become external ; and in 

 Nereis and Aphrodita are furnished with a number of muscles, 

 are evidently articulated, and become organs of reptation 

 already very perfect. Lastly, in the Myriapoda, the integu- 

 ments are become solid, the feet present well-defined articu- 

 lations, but the pieces of which they are composed are all 

 nearly similar; and it is only when we arrive at Dorsigera, that 

 we find the feet assume the form so conspicuous in insects. 



Arrived at the Myriapoda, the feet decrease in order of rank. 

 The posterior diminish {s atroplnenf) in passing to Lepisma, 

 where the greater number are only rudimentary; the three pair 

 belonging to the three segments of the trunk retain their usual 

 form, and maintain it to the end of the Class Lisecta. 



Already, in Scolopendra, the last pair of legs, though much 

 stronger than the others, no longer serve for locomotion ; and 

 in Lepisma they are changed into the long filaments at the 

 extremity of the abdomen, which are again found in some 

 Orthoptera and Neuroptera. 



Li the Crustacea, the feet likewise undergo a change of this 

 nature ; but as they also experience a change of function, they 

 will be spoken of as examples of the next law. 



Third Law. — The organs often change function to replace 

 others which disappear. 



It is very rare that nature introduces a new organ, if another, 

 become useless, can fulfil the same conditions. This admirable 

 economy is remarkable, even in the forms which are preserved 

 as long as possible; and the parts of the first system {appareil) 

 which no longer are of service to the new function only, dis- 

 appear gradually, by becoming more and more rudimentary. 

 These parts, which are then as it were superadded to the 

 organs, serve very advantageously to recognize the primitive 

 use of these last. 



Of this, the feet again serve for an example : in Aphrodita 

 they are all locomotors ; whilst in Scolopendra the first pairs 



