448 EMBRYOLOGY. Chap. XIII. 



resemble the mature parent-form. We have seen that 

 this is the rule of development in certain whole groups 

 of animals, as with cuttle-fish and spiders, and with a 

 few members of the great class of insects, as with Aphis. 

 With respect to the final cause of the young in these 

 cases not undergoing any metamorphosis, or closely 

 resembling their parents from their earliest age, we 

 can see that this would result from the two following 

 contingencies ; firstly, from the young, during a course 

 of modification carried on for many generations, having 

 to provide for their own wants at a very early stage 

 of development, and secondly, from their following 

 exactly the same habits of life with their parents ; for 

 in this case, it would be indispensable for the existence 

 of the species, that the child should be modified at a 

 very early age in the same manner with its parents, in 

 accordance with their similar habits. Some further 

 explanation, however, of the embryo not undergoing 

 any metamorphosis is perhaps requisite. If, on the other 

 hand, it profited the young to follow habits of life in any 

 degree different from those of their parent, and conse- 

 quently to be constructed in a slightly different manner, 

 then, on the principle of inheritance at corresponding- 

 ages, the active young or larvae might easily be ren- 

 dered by natural selection different to any conceivable 

 extent from their parents. Such differences might, 

 also, become correlated with successive stages of deve- 

 lopment; so that the larvae, in the first stage, might 

 differ greatly from the larvae in the second stage, as we 

 have seen to be the case with cirripedes. The adult 

 might become fitted for sites or habits, in which organs 

 of locomotion or of the senses, &c, would be useless ; 

 and in this case the final metamorphosis would be said 

 to be retrograde. 



As all the organic beings, extinct and recent, which 



