CONSIDERATIONS. 7 



these as are common to all the organs, M. Straus-Durckheim 

 next proceeds, leaving the others to be examined in those parts 

 of the work in which each system is treated of separately. 



First Law. — The organs, at one extremity of the scale, 

 exercise always a very evident function ; whilst, at the other 

 extremity, they are constantly rudimentary and without func- 

 tion, and at last disappear. 



We may here distinguish two cases. In the first, the organs 

 present themselves at the head of the scale, developed to the 

 highest degree of which they are susceptible, and decrease 

 insensibly, until they arrive at the other extremity of the scale. 

 Thus, for example, the posterior wings of insects are intro- 

 duced suddenly into their organization at the highest point of 

 perfection in the Coleoptera, where they alone serve for flight. 

 In the Orthoptera and Hemiptera, they already begin to divide 

 their function with the elytra ; and go on diminishing gradually, 

 until they exist merely as rudiments in the Diptera, where they 

 are represented by the Hal teres : finally, they entirely disap- 

 pear in the Aphaniptera. 



In the second case, the organs do not appear at the head of 

 the scale in their most perfect state of development, and only 

 acquire it by degrees. Such is the case with the elyti-a, or 

 first pair of wings, which follow a course exactly contrary to 

 the inferior wings. They only appear, at first, as organs 

 slightly accessory to flight. In this state they remain nearly 

 throughout the whole of the Coleoptera. In the Orthoptera 

 and Hemiptera, they begin already to take a very active part in 

 flight, but still preserve their primitive use. In the Neuroptera, 

 particularly the Libellulina, they attain the development of the 

 anterior wings, from which they differ but very slightly. From 

 this point, they continue to surpass them, until, in the Diptera, 

 they become the sole organs of flight, and, of course, hkve 

 attained their highest degree of perfection. Arrived at this 

 culminating point, they suddenly diminish in Hippobosca, and 

 entirely disappear in Pulex. 



Although, in this last case, the organs present their most 

 perfect degree towards the middle of the scale, and decrease 

 towards each extremity ; yet, they never are found at the head 

 of the scale in that state of perfect nullity which can properly 

 be called rudimentary, but serve, more or less directly, for tlie 

 function to which they are destined. 



