402 THE ORIGIN OF SPECIES 



influence long continued use or disuse may have had in modifying 

 the limbs or other parts of any species, this will chiefly or solely 

 have affected it when nearly mature, when it was compelled to use 

 its full powers to gain its own living; and the effects thus produced 

 will have been transmitted to the offspring at a corresponding 

 nearly mature age. Thus the young will not be modified, or will be 

 modified only in a slight degree, through the effects of the in- 

 creased use or disuse of parts. 



With some animals the successive variations may have super- 

 vened at a very early period of life, or the steps may have been 

 inherited at an earlier age than that at which they first occurred. In 

 either of these cases, the young or embryo will closely resemble 

 the mature parent-form, as we have seen with the short-faced 

 tumbler. And this is the rule of development in certain whole 

 groups, or in certain sub-groups alone, as with cuttle-fish, land- 

 shells, fresh-water crustaceans, spiders, and some member of the 

 great class of insects. With respect to the final cause of the young 

 in such groups not passing through any metamorphosis, we can 

 see that this would follow from the following contingencies: 

 namely, from the young having to provide at a very early age for 

 their own wants, and from their following the same habits of life 

 with their parents; for in this case it would be indispensable for 

 their existence that they should be modified in the same manner as 

 their parents. Again, with respect to the singular fact that many 

 terrestrial and fresh-water animals do not undergo any meta- 

 morphosis, while marine members of the same groups pass through 

 various transformations, Fritz Miiller has suggested that the 

 process of slowly modifying and adapting an animal to live on the 

 land or in fresh water, instead of in the sea, would be greatly sim- 

 plified by its not passing through any larval stage; for it is not 

 probable that places well adapted for both the larval and mature 

 stages, under such new and greatly changed habits of life, would 

 commonly be found unoccupied or ill-occupied by other organisms. 

 In this case the gradual acquirement at an earlier and earlier age 

 of the adult structure would be favored by natural selection; and 

 all traces of former metamorphoses would finally be lost. 



If, on the other hand, it profited the young of an animal to fol- 

 low habits of life slightly different from those of the parent-form, 

 and consequently to be constructed on a slightly different plan, or 

 if it profited a larva already different from its parent to change 

 still further, then, on the principle of inheritance at corresponding 

 ages, the young or the larvae might be rendered by natural selec- 

 tion more and more different from their parents to any conceivable 



