250 DEVELOPMENT AND E:\IBRY0L0GY. [Chap. XIV. 



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, throucrh the effects of the increased use or 

 disuse of parts. 



With some animals the successive variations may 

 have supervened 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 members 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 contin- 

 gences ; 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 tliis 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 metamorphosis, whilst 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 

 simplified 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 



