•394 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



would follow from the following contingences ; 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 metamorphosis, whilst 

 marine members of the same groups pass through various transfor- 

 mations, 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 greatly changed habits of life, would commonly be found un- 

 occupied or ill-occupied by other organisms. In this case the 

 gradual acquirement at an earlier and earlier age of the adult 

 structure would be favoured 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 follow 

 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 larva; might be rendered by natural selection more 

 and more different from their parents to any conceivable extent. 

 Differences in the larva might, also, become correlated with succes- 

 sive stages of its development ; so that the larva, in the first stage, 

 might come to differ greatly from the larva in the second stage, as 

 is the case with many animals. The adult might also become fitted 

 for sites or habits, in which organs of locomotion or of the senses, 

 ,&c., would be useless ; and in this case the metamorphosis would be 

 retrograde. 



From the remarks just made we can sec how by changes of struc- 

 ture in the young, in conformity with changed habits of life, to- 

 gether with inheritance at corresponding ages, animals might come 

 to pass through stages of development, perfectly distinct from the 

 primordial condition of their adult progenitors. Most of our best 

 authorities are now convinced that the various larval and pupal 

 stages of insects have thus been acquired through adaptation, and 

 not through inheritance from some ancient form. The curious case 

 of Sitaris — a beetle which passes through certain unusual stages of 

 development — will illustrate how this might occur. The first larval 

 from is described by M. Fabre, as an active, minute insect, furnifihed 



