380 EVOLUTION, OLD AND NEW. 



" The great development of mantle in the acephalous 

 molluscs has rendered eyes, and even a head, entirely 

 useless to them. These organs, though belonging to 

 the type of the organism, and by rights included in it, 

 have had to disappear and become annihilated owing 

 to continued default of use. 



****** 



" Many insects which, by the analogy of their order 

 and even genus, should have wings, have nevertheless 

 lost them more or less completely through disuse. A 

 number of coleoptera, orthoptera, hymenoptera, and 

 hemiptera give us examples, the habits of these animals 

 never leading them to use their wings." * 



I will here bring this present volume to a conclusion, 

 hoping, however, to return to the same subject shortly, 

 but to that part of it which bears upon longevity and 

 the phenomena of old age. In * Life and Habit ' I 

 pointed out that if differentiations of structure and 

 instinct are considered as due to the different desires 

 under different circumstances of an organism, which 

 must be regarded as a single creature, though its 

 development has extended over millions of years, and 

 which is guided mainly by habit and memory until 

 some disturbing cause compels invention then the 

 longevity of each generation or stage of this organism 

 should depend upon the lateness of the average age of 

 reproduction in each generation ; so that an organism 

 (using the word in its usual signification) which did not 

 upon the average begin to reproduce itself till it was 



* 'Phil. Zool.,' torn. i. p. 245. 



