xiiii.] THE CAUSES OF DEVELOPMExNT. 301 



whicli arise by budding from the skin of tlie embryo.^ 

 For if embryology is an index of descent (and tliis, I think, 

 cannot be reasonably doubted), we mu.st suppose that the 

 development of the individual repeats, in a short time, the 

 development which occupied untold generations of the 

 species. It can scarcely be necessary to repeat, that all 

 actions whatever, tending to modify the nature of an 

 organism, will accumulate their effects through successive 

 generations. 



I will now go on to mention two remarkable instances, 

 in which organs originally employed for another purpose 

 have become converted into respiratory organs. These 

 two instances include the principal, though not the only 

 classes of air-breathing animals. 



The leech and the earthworm have two small sacs in Homo- 

 each segment opening on the external surface of the body. thTi.e.siji- 

 The function of these appears to be to secrete muci;s. ratory 

 But there is a gradation, through the Millepedes andinfe^cte-* 

 Centipedes, between these sacs and the air-tubes or tra- 

 cheae by which insects respire. ^ The tracheae of the insect 

 are consequently homologous with the mucus-sacs of the 

 worm ; and, if the development theory is true, the sacs, 

 being no longer needed for the purpose of secreting mucus, 

 have, through successive generations, changed the character 

 of their lining, and have extended through the body, so as 

 to become transformed into respiratory organs, adequate to 

 meet the wants of such active animals as insects. What 

 made the transformation possible, was the fact that every 

 surface exposed to the air or the water in which the animal 

 lives is (if not too much indurated, like the shell of a tor- 

 toise or of a crab) in some degree a respiratory surface. ^ 



The wings of insects are due to a transformation Avhich 

 is even more wonderful than this Unlike the wings of 

 Pterodactyles, birds, and bats, they are in no way homo- 

 logous with legs ; on the contrary, they are now generally 

 believed to be appendages of the respiratory system ; the 



^ Spencer's Principles of Biolog}-, vol. ii. p. 293, 

 2 Carpenter's Comparative Pliysiology, p. 748. 

 Carpenter's Human Physiology, p. 293. 



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