370 DEVELOPMENT AND EMBRYOLOGY. [CHAP. XIV. 



many terrestrial and fresh-water animals do not undergo any 

 metamorphosis, whilst marine members of the same groups pass 

 through various transformations, Fritz Muller 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 unoccupied or ill-occupied by 

 other organisms. In this case the gradual acquirement at an 

 earlier ana 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 corre- 

 sponding ages, the young or the larvae 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 successive 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 see how by changes of 

 structure in the young, in conformity with changed habits of life, 

 together 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 form is described by M. Fabre, as an 

 active, minute insect, furnished with six legs, two long antennae, 

 and four eyes. These larvae are hatched in the nests of bees ; and 

 when the male-bees emerge from their burrows, in the spring, 

 which they do before the females, the larvae spring on them, and 

 afterwards crawl on to the females whilst paired with the males. 

 As soon as the female bee deposits her eggs on the surface of the 

 honey stored in the cells, the larvae of the Sitaris leap on the eggs 

 and devour them. Afterwards they undergo a complete change \ 



